Version: 25.2.2.1 (X86_64) / LibreOffice Community
Build ID: 38d746d66d9b82fa248a2e90142b9dd3ddd1d6cd
CPU threads: 8; OS: Linux 6.8; UI render: default; VCL: gtk3
Locale: de-DE (de_DE.UTF-8); UI: en-US
Calc: threaded

and

Version: 26.2.0.0.alpha0+ (X86_64) / LibreOffice Community
Build ID: 480ef73deef62c458e5735cd496a1d74ef408ed8
CPU threads: 8; OS: Linux 6.8; UI render: default; VCL: gtk3
Locale: sv-SE (de_DE.UTF-8); UI: de-DE
Calc: threaded

Next to the underline button, there is a small downward triangle that allows you to select types of underlining. This feature is also available for the buttons for colored font, colored font background, etc.

When you make a selection using these other direct formatting buttons, the choice remains preset for the next time you use that button.

Inconsistently, this is not the case with the various underlining styles.
The button function should be standardized for direct formatting: The easy-to-remember rule for such a button with an extension selection is usually that the selected style remains in effect until further notice.

How to reproduce

1. Open a LO Writer document
2. Write at least two words into it.
3. Select the first word
4. Select the downward triangle adjacent to the underline tool button
5. Select the fat dotted underline style
Result: the word becomes underlined accordingly, as expected. The tool button appears in the activated style. (The U on the Button should reflect the type of underlining to its best however).

6. Select the another Word
Result: The underline button appears without highlight (if the word was not underlined before) – as expected.

7. Click on the button
Result: normal simple underlining. I would have expected the last previously set underlining type.

The tool buttons with a sub-selection might not all executed by the same code in the GUI. Then you should clean up: One function for all buttons with sub-selection. When you select it: Display the options (selection from a list or special thing like the color selection tools). When the selection is made: Display it on the button, apply the marking type, and display the button as pressed.

I see about 10 tool buttons with sub-selections that can be standardized. Those for color selections for the foreground or background are slightly different in their sub-function: The sub-selection must be done using a procedure variable, which, as in the other cases, can return an integer. For the other functions, this would be the ordinal number from the drop-down sub-selection. However, there are also advanced buttons that do not trigger a “permanent function” and therefore do not need to change the button appearance. Perhaps only the wrong type was selected for the underlining tool button.