No, not normal but happens all the time. Like you I once replace a toilet
with 4 new flappers and they all still leak.
Get the flapper with the sticky-back seat combo (fluidmaster has them) -
works for me most of the time.
You could pour hot water into the empty tank and let the rubber flapper
conform to the seat. Works some of the time. I think, from my experience,
this works better than rubbing vasoline on the back of the flapper which
works, if at all, during the first few flushes.
I had one that was driving me nuts. Turns out the top portion of the
overflow plastic tube was cracked and lowering the water level stop the
leak. This one will be retired with a new toilet and new floor when I have
I also had hairline cracks on the tanks a couple of times but you will see
some water on the outside tank, on the floor or around the toilet. My
solution was to replace with a new toilets.
Then there are toilets that would not cooperate no matter what I do. Good
thing is the low end toilets are cheap and works well and could be changed
within an hour.