About the shove resistance check, I looked carefully everytime, and there wasn't a Save dice icon appearing.
My fighter is a 16 STR human with proficiency in Athletics.
I attempted that encounter 5 times, and even when I was hugging the wall behind the sleeping bugbears, they could find a way to shove me down the ledge.
I calculated the distance from my character to the ledge, not including the falling distance.
There's no clear indication but not all shoves succeed and in my experience, which obviously isn't matching yours, characters with more athletics resist shoves more often than characters with less. Also, some characters have way more luck pushing enemies than others. And the shove distances seem to be affected by strength. My strength 8 warlock could barely make anything move a few meters, but Laezel can put her back into it.
As for your high strength human going flying like that, it sounds ridiculous. Definitely not my experience. If it is happening that reliably, quicksave just before it happens and report it along with the save file, I would say.
