There are two very small sensors at the very top left of the screen. If either are covered (by a screen protector or even a thumb), the phone thinks it I'd still held to your ear and the screen won't turn back on.
There is a bug here. I'm experiencing the same issue on my Pixel 3 (Google fi) with no screen protector and when I take the case off. If I place a call with the speaker activated things go well. As soon as I switch off speaker phone the screen turns off and won't turn back on until the other party hangs up the call.