You could check if the ESC key works at all by shorting the two pins, for example with an unfolded paper clip (non-insulated one of course), or some other wire you may have access to. Only if that works, you have a chance for proper operation with a new membrane.
That said, some A500 keyboards don't even use a membrane. The early ones have individual switches, and the ESC key could just have a cold solder joint.
Been too lazy and busy to have a look at my keyboard until now. The keyboard I use at the moment only had one key not registering. It's nr 9 (under the F keys). And the problem was the rubber thingy inside the key.
9 didn't work, no matter how hard I pressed, so I thought maybe the membrane had a broken connection somewhere, but used some tweezers (while wearing a rubber glove ofc, just in case) and noticed there's nothing wrong with the membrane as 9 registers when the tweezers touches them. So, I looked at the rubber thingy inside/under the key and cleaned its tips on the side gently with some IPA and it still wouldn't work. I pushed it against other keys and none would register with nr 9's rubber thingy. So I took out a rubber thingy from another key and used it on nr 9 on the membrane and it worked. So, there's something wrong with 9's rubber thingy.
Only difference I can see between key 9's rubber thingy and other other keys is it's faded in color. I see no breakage or anything else. The tips might be a bit worn out maybe.
So, if they are faded they don't work?
Attached some images.
9 is more important than the key to the left (see image), so swapped the rubber thingy with that one. So 9 works now, but ofc not that other key, which is OK.
Edit: OK, seems the "plunger", as they are called, has a conductive coating on them and key 9 has it completely worn off and needs to be recoated or replaced. So no IPA should be used on it of course, which I did, but it was already coatless. Still wonder why only that key is like that, and the color is faded, while the others are not.
Edit 2: Ordered 3 new plastic bits with plungers on them from ebay. One for the broken key, one for F3 which works but have to press really hard, and one more in case I might need it in the future.