I just discovered this is a very limited problem that only affects me on an iPad Pro 12.9” (second gen) with an Apple Smart keyboard.
If I remove the smart keyboard and use the on-screen keyboard, pressing enter on the search field filters the list as expected.
Same on my macbook air, it seems to work fine.
Only the combination of the iPad Pro + Apple Smart Keyboard doesn’t work properly. However, if I tap the magnifying glass after entering a search value, the list is filtered.
So basically there is something different about how the web interface is handling a return from a smart keyboard versus a return from the on-screen keyboard. I can’t imagine they would send anything different, but one works and the other doesn’t.
The iPad Pro with the smart keyboard doesn’t have problems with carriage returns anywhere besides SmarterMail, whether in the browser or in an app. It DOES however have problems in other SmarterMail search fields though. I confirmed the same problem exists with the Spool search fields as well.
Must be input filtering code on those fields that is somehow acting differently between an iPad on-screen keyboard and input from a smart keyboard attachment.