As for existing sheet with content, I could reproduce sometimes but not always, maybe depending on the content or if clicked outside the table before Undo. 
That should be also tested. 

There's another Undo problem, but not from the referenced commit as it was reproducible also before (to be tested separately):
Open a new calc sheet
Mark some cells
Apply borders to the cells: Criss-Cross Border
Undo to remove the borders
Wrong behavior: Borders are half removed (slashed)

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Version: 7.6.0.0.alpha0+ (X86_64) / LibreOffice Community
Build ID: 37e3455a13ab5741104bf41d05a80e60a4612682
CPU threads: 4; OS: Windows 6.3 Build 9600; UI render: Skia/Raster; VCL: win
Locale: nl-NL (nl_NL); UI: en-US
Calc: CL threaded

(In reply to Timur from comment #1)
> As for existing sheet with content, I could reproduce sometimes but not
> always, maybe depending on the content or if clicked outside the table
> before Undo. 
> That should be also tested. 

I have the same exact behavior. It is not exactly in each and every situation, but I wasn't able to determine in which case(s) it doesn't happen.

What I _can_ say is that when I undo, the relevant cell(s) look as if they were "selected", while the focus is still elsewhere (but this is probably not a surprise):

1. Click once on one cell.
2. Apply one outer border combination (it doesn't have to be all the 4 sides).
3. Click on some other cell.
4. [CTRL]+[Z]
5. Note that the border is still on, the cell looks as if it was "selected", while the focus is on the second cell (as demonstrated by the cell address shown on the top-left box between column A and row 1).

> 
> There's another Undo problem, but not from the referenced commit as it was
> reproducible also before (to be tested separately):
> Open a new calc sheet
> Mark some cells
> Apply borders to the cells: Criss-Cross Border
> Undo to remove the borders
> Wrong behavior: Borders are half removed (slashed)

In the case of diagonal lines, it varies depending on how many cells I select to apply the border attribute. For one cell only, I can apply both diagonal borders at _once_, and then undo _twice_ (yes, 2 actions are available), effectively going back to the status before applying both diagonal lines. But when the lines are applied simultaneously to several cells (also at once), the first undo works whereas the second doesn't – it is available, but it doesn't undo the "second" diagonal.

*** Bug 154066 has been marked as a duplicate of this bug. ***

Original issue resolved already in 7.6.
Comment 1 still in 26.2+ but that is separate.