@Telesto -- now you can confirm my extension of your report.

0. Open New Document; get to Transparency tab of a Paragraph Style dialog.
   Result:  Notice that preview is "blank"
1. Switch to Area tab, select any tab (except for none), notice Preview.
2. Switch back to Transparency.
   Result:  the selection shown in "Preview" in Area is shown as "Preview"
            in Transparency (for any of the non-None tabs in Area).
3. Press "Reset"
   Result:  the Preview is "blank"  (this happens for all non-"None" tabs
            in Area.  [this confirms the OP and extends it to all non-"None"
            tabs in Area]

Now go further to understand the source of the problem.

4. Switch to Area tab, select a tab (e.g., Pattern) and press "Apply"
5. Now repeat steps 2 and 3.
   Result:  the preview in Transparency will "reset" to what was "applied" 
            in the Area tab (e.g., the pattern if "Pattern" was used).

In sum:  "Reset" in "Transparency" resets to the "last applied" Area.  When nothing has been saved for Area, it "resets" to "blank" (which is what motivated the present bug report).  If something was applied in the Area tab, then the
Transparency tab will "reset" to that value, regardless of which tab is currently selected in Area.

However -- if you use "Apply" in Transparency -- with an "unapplied" Area value, then *both* the "Area" and the "transparency" are applied. 

IOW, the transparency tab is inviting/allowing you to "test" transparency values with an *unapplied* area value, where both can be saved simultaneously with "Apply" in Transparency.  When you want to "reset" the Transparency tab to get back to where you started -- then "transparency" does reset its values, but it also "resets" the Area in the preview to the currently saved value for Area.

Probably what is needed is for the "preview" field for "transparency" to not "reset" to the currently saved Area value, but to use whatever is currently selected in Area, even if not saved.  That would respect the affordances of the interface.

Tested with: 
Version: 7.2.0.0.alpha0+ (x64) / LibreOffice Community
Build ID: e60bebd4c5257b0f592d27c74399de1498ac725b
CPU threads: 8; OS: Windows 10.0 Build 19041; UI render: Skia/Raster; VCL: win

(In reply to sdc.blanco from comment #1)

Sorry - important clarification - to more readily see the effect. 

> 4. Switch to Area tab, select a tab (e.g., Pattern) and press "Apply"
> 5. Select another Area tab (e.g., Gradient)
> 6. Now repeat steps 2 and 3.
>    Result:  the preview in Transparency will "reset" to what was "applied" 
>             in the Area tab (e.g., the pattern if "Pattern" was used).

Reproduced as described in description and comment 1 (although I don't have an Apply button), with:

Version: 7.6.0.0.alpha0+ (X86_64) / LibreOffice Community
Build ID: 8ae84bb5566e12df64236a116b9d1889d6f5f052
CPU threads: 8; OS: Linux 5.15; UI render: default; VCL: gtk3
Locale: en-AU (en_AU.UTF-8); UI: en-US
Calc: threaded

This same "resetting of settings outside the transparency tab" is seen elsewhere, e.g. in the Area dialog for a shape, the Page style dialog, etc.
Hard to pick a couple of meta bugs...

We have 2 kinds of reset:
- Reset: "Unsaved modifications to this tab are reverted"
- Reset to Parent: "Values on this tab in Contains in Organizer are removed"

So, I think that it is correct NOT to have a preview.
Before we didn't have a preview, after the reset it is the same. So, everything is fine.

Version: 24.8.0.3 (X86_64) / LibreOffice Community
Build ID: 0bdf1299c94fe897b119f97f3c613e9dca6be583
CPU threads: 4; OS: Linux 6.8; UI render: default; VCL: gtk3
Locale: ro-RO (ro_RO.UTF-8); UI: en-US
Calc: threaded