I started using PopClip a few months ago and find it incredibly useful. I also use ClipboardHistory, one of the numerous clipboard managers available.
When I first started using PopClip I do not remember selected text being automatically copied to the clipboard. My main uses of PopClip are changing capitalization, removing spaces, or adding hyphens. Recently I started noticing my ClipboardHistory being littered with these bits of text that I had applied PopClip actions to.
In another thread Nick implies this is a bug in MacOS (I’m on Catalina). The only update I may have done recently was maybe a security update or something minor, not a system upgrade. Sequential Paste just FASTER! - #4 by nick
I would like for this not to happen if possible, but it’s an annoyance, not a dealbreaker. I could see it being problematic on for someone not using a clipboard manager because it would change the sole contents of the clipboard.