Sounds like it has gotten confused about underline settings (assuming underline isn't turned on).
I think you'd have to use "load styles from template" to reset the styling (from memory this is under the Styles menu but been a while since I've had to go digging); but this may mean more work elsewhere putting other formatting back as needed.
Something has gone wrong I think with the settings with what I am working on, could well be the styling. I am going to have to see how to reset the styling for this particular document.I would just uninstall and reinstall it tbh.
It will carry styling metadata unless you set it to paste as per destination formatting. Pasting it in to something with only one style for the whole document like notepad would also strip said metadataSomething has gone wrong I think with the settings with what I am working on, could well be the styling. I am going to have to see how to reset the styling for this particular document.
I have done a reset by going into help and safe mode it seems to be behaving normally if I'm doing anything new, but if I copy and paste all the work I have done into a new writer page it then happens again, so it looks like a messed up setting within the document I'm working on rather than writer itself.