Hello and thank you for the report.  I'm can confirm the behavior with

Version: 7.5.6.0.0+ (X86_64) / LibreOffice Community
Build ID: f0e825382a76d685998be702ed551a00b73476a5
CPU threads: 7; OS: Linux 6.5; UI render: default; VCL: gtk3
Locale: en-US (en_US.UTF-8); UI: en-US
Calc: threaded

and

Version: (X86_64)/LibreOffice Community
Build:   9f56dff12ba03b9acd7730a5a481eea045e468f3
Environment: CPU threads: 16; OS:Windows 10.0 Build 22000
User Interface: UI render: Skia/Vulkan; VCL: win
Locale:  en-US (en_US); UI: en-US
Misc:    Calc: CL threaded

The blue line disapears after the table is added, but I was able to get it to reappear by moving the table down one line(hit enter with cursor in the table). I could then remove the page break by backspacing.  

Do you get similar behavior?

Setting to NEEDINFO

Yes, if you press Return in the very first table cell, the table moves down one row and the dashed line is visible again.

There seems to be other strange behavior (broken redo log) with tables and page columns:

On an empty document:

1. Apply "Columns: 2" to the "Default Page Style"
2. insert table (Ctrl+F12 and Return)
3. insert col break (Ctrl+Shift+Return) (seemingly nothing happens)
4. insert new paragraph (Return)
5. insert col break (Ctrl+Shift+Return) 
6. remove first line (Del Key)


The dashed line (which indicates a column break) is also hidden here. But now the redo log seems to be broken. The following actions are displayed when you move the mouse pointer over the undo button after performing each of the above actions.

1. Undo: Change style: Default Page Style
2. Undo: Insert table: "Table1"
3. Undo: Change style: Table1
This logged action seems wrong to me.
4. Undo: New Paragraph
5. Undo: Insert column break
6. Undo: Delete Paragraphs


Pressing the undo button from the toolbar (or Ctrl+Z, similar), the following things happen:

1. Undo: Delete Paragraphs
The Table disappears, the man. col break appears, the removed pilcrow appears (Formatting Marks shown).
2. Undo: Insert column break
The Table appears, the man. col break disappears, the pilcrow disappears. Table remains in second col.
3. Undo: New Paragraph
The table moves to the first page.
4. Undo: Change style: Table1
seemingly nothing happens
5. Undo: Insert table: "Table1"
The table disappears
6. Undo: Change style: Default Page Style
The Document changes to one column.


Redo the undone things results in another result than before:
6. Redo: Change style: Default Page Style
The Document changes to two columns.
5. Redo: Insert table: "Table1"
The table appears
4. Redo: Change style: Table1
seemingly nothing happens
3. Redo: New Paragraph
The table remains on the first page and moves one line down
2. Redo: Insert column break
The Table moves to the second col, second line, and the col break dashed line is shown
1. Redo: Delete Paragraphs
The Table moves back to the first col, the dashed line goes away


Maybe, this should be placed in a new issue?

Setting to NEW with the caveat that there is an easy work around, while the cursor is in the first row and column of the table, press enter to move the table down one line and the page break re-appears.

(In reply to demo4 from comment #2)
> Yes, if you press Return in the very first table cell, the table moves down
> one row and the dashed line is visible again.
> 
> There seems to be other strange behavior (broken redo log) with tables and
> page columns:
> 
> On an empty document:
> 
> 1. Apply "Columns: 2" to the "Default Page Style"
> 2. insert table (Ctrl+F12 and Return)
> 3. insert col break (Ctrl+Shift+Return) (seemingly nothing happens)
> 4. insert new paragraph (Return)
> 5. insert col break (Ctrl+Shift+Return) 
> 6. remove first line (Del Key)
> 
> 
> The dashed line (which indicates a column break) is also hidden here. But
> now the redo log seems to be broken. The following actions are displayed
> when you move the mouse pointer over the undo button after performing each
> of the above actions.
> 
> 1. Undo: Change style: Default Page Style
> 2. Undo: Insert table: "Table1"
> 3. Undo: Change style: Table1
> This logged action seems wrong to me.
> 4. Undo: New Paragraph
> 5. Undo: Insert column break
> 6. Undo: Delete Paragraphs
> 
> 
> Pressing the undo button from the toolbar (or Ctrl+Z, similar), the
> following things happen:
> 
> 1. Undo: Delete Paragraphs
> The Table disappears, the man. col break appears, the removed pilcrow
> appears (Formatting Marks shown).
> 2. Undo: Insert column break
> The Table appears, the man. col break disappears, the pilcrow disappears.
> Table remains in second col.
> 3. Undo: New Paragraph
> The table moves to the first page.
> 4. Undo: Change style: Table1
> seemingly nothing happens
> 5. Undo: Insert table: "Table1"
> The table disappears
> 6. Undo: Change style: Default Page Style
> The Document changes to one column.
> 
> 
> Redo the undone things results in another result than before:
> 6. Redo: Change style: Default Page Style
> The Document changes to two columns.
> 5. Redo: Insert table: "Table1"
> The table appears
> 4. Redo: Change style: Table1
> seemingly nothing happens
> 3. Redo: New Paragraph
> The table remains on the first page and moves one line down
> 2. Redo: Insert column break
> The Table moves to the second col, second line, and the col break dashed
> line is shown
> 1. Redo: Delete Paragraphs
> The Table moves back to the first col, the dashed line goes away
> 
> 
> Maybe, this should be placed in a new issue?

Yes, file this as another issue as its not directly related to the current issue.

Bug is still present in 

Version: 25.8.3.2 (X86_64)
Build ID: 8ca8d55c161d602844f5428fa4b58097424e324e
CPU threads: 12; OS: Windows 10 X86_64 (build 19045); UI render: Skia/Raster; VCL: win
Locale: de-DE (de_DE); UI: de-DE
Calc: CL threaded