Update: Lol... Right after I posted my previous comment, I received an email reply from AdSense about the invalid click activity. So forget everything in my previous post. :)
The email was fairly generic, stating that the clawbacks are due to accidental clicks on ads, and it offered the usual advice on how to reduce the risk of accidental clicks (proper spacing around ads, keeping ads away from navigation buttons, etc.).
However, the person who emailed me back was also able to tell me which ad unit is most responsible for the highest invalid click activity, and that most of it is coming from high-end mobile devices. So I'll be taking a closer look at it and seeing if I can figure out what's going on. I'm still not sure why this has suddenly become an issue. I've used the same ad layouts for a long time without any problems.
Overall, it was a very friendly email. No threats of closing my account or anything like that. Just some helpful advice. I'm glad I emailed them now.
[edited by: Cralamarre at 4:01 pm (utc) on Apr 2, 2019]