Glad to know I’m not just gaslighting myself and this is a common experience 
Seems like the issue is with the reaction plugin, which agrees with how long I’ve been experiencing this problem on meta. My forum does not use the reaction plugin so another piece of evidence.
Ultimately this issue isn’t affecting my forum but I can say it’s pretty jarring find a post I accidentally liked here without realizing, combined with the default setting that prevents unlikes after a certain period of time.