This is a problem that appears intermittently. I used to get it with my previous phone (Nexus 5) and had hoped that when I upgraded to a Pixel 3a it would be a thing of the past - but no! What happens (from time to time) is that the wifi icon indicates strong wifi but no internet. As a result the phone starts using 4g data for internet connections. However, if I turn off 4g data I can access all internet sites over wifi without any trouble, so the wifi does, in fact have internet access although the phone says it doesn't. This is annoying because if I don't spot it and turn off 4g data it starts to waste my 4g data allowance. After some variable amount of time (usually up to an hour) it suddenly reverts to normal. I haven't been able to find any answer to this (or indeed anyone reporting the same problem) even though I've been searching off and on for several years.