search results now underlines entire paragraph (in addition to yellow highlighting the term)

I assume that was your topic stylesheet? The properties look rather like something used for code samples. So if you have those in your help, that's where I'd check for issues. I might be able to help suggest workarounds if you find it is needed.

This could also happen if you've set the span tag in your topic stylesheet to be underlined (although if this was the case I'd actually expect you'd see a lot of randomly underlined text in your topics, for reasons I won't go into here).

You should also be able to find out where the underline comes from using the Developer tools. Right-click on the underlined text and select Inspect (for Chrome - other browsers have the same functionality but the wording may be a bit different). The right-hand panel should show the style definition for the selected element.

I don't recall anyone ever mentioning that happening in RH2022 - is it also happening in the sample project? If so, you should report it to the RH folks (the forum isn't the place to do that) at https://helpx.adobe.com/contact/enterprise-support.other.html#robohelp for your Adobe Support options. I'd recommend using the tcssup@adobe.com e-mail address as it reaches a team dedicated to Technical Communication Suite products including RoboHelp. Or by using the Tracker (https://tracker.adobe.com/) to create a bug. I'd go with the first route to start with.