I also encountered this after updating XFCE 4.14 to 4.16 today with the Manjaro stable update. Here's how it now looks:
I drew the red box to show the space that xfce4-sensors-plugin is now taking.
I also have this running in a vertical panel on the left. I had previously had "Number of text lines" set to 10. However, even if I set "Number of text lines" to 1, the issue is not resolved.
I've tested again today with a build from the latest revision and this issue is still relevant. I see there's a merge request from @stergem to fix this. Can that be reviewed and/or merged soon? My desktop is pretty messed up from this issue. Thanks so much for reading this!