Thanks for taking time writing the post.
I do have the same problem regularly but not consistently -- it's hard to replicate what triggers it. So far I can describe a behaviour I experience: while inputting search terms, it does search each term automatically (without the need to press enter). Sometimes the search "lags" a bit and clears the search box.
I do realise, it's unclear what "sometimes" and "lag" means in this context and when I got time I'll try to collect logs, screenshots and more obvious steps to replicate it.
Last time I stumbled upon it in this environment:
Joplin 1.8 (prod, linux) yes, not the latest
Also I think I've seen it on Windows 7 but I cannot say reliably what version of Joplin.