I have tracked this problem down. Apparently (according to Chatbot Claude) there can be complicated interactions between Qt and Windows as to which one is going to point various components such as radio boxes. Claude claims that there can be subtle bugs in this cooperation, and they could kick in with a dark theme vs light theme.
I added styling specifically for radio button indicators to two dark style sheets, and now the Find panel buttons work as expected. The CSS selectors involved are ones like "QRadioButton::indicator:checked" - there are three similar selectors. I don't think any of the current theme files have them. We must have been lucky in the past!
Now I have to make corresponding changes to all my dark themes and also the Leo's standard dark themes. When I have got them all working (let's hope that works) I'll submit a PR.