Seems to me that the pertinent issue is “why does the dialer Notification not appear over other content … or is it hidden or disabled?”. There are multiple other pointers in those threads, so please study them.
Just for starters, is the behaviour seen in Safe mode?
Phone running, press Power button > Long press Power off icon.
Edit after rapid reaction below.
This is not behaviour which I have ever seen in four different /e/OS phones.
My experience is: when the screen is already on and rings, swipe down from the upper edge of the screen. Then you see a little phone window on which you have the familiar “accept” and “deny” buttons. Alternatively, configure a remaining key like “volume” to accept calls.
As this situation happend first time to me, I was very confused too. I think, I lost that call and had to call back.
I come from BlackBerrys which had dedicated keys for that function. But today every gadget, incl. cars have to be “all touch” to be fancy … and softkeys may disappear.