My issue similar to this on the Giant was resolved because Giants are not supported and I didn't know that at the time. Since Giants now use Zombie AI, I checked to see if Zombies also phased through the platform, and it did. I repeated the process with both Giants AND Zombies (which both fell through every time), summoned a Drowned and a Husk (neither of which fell through), and even landed on the platform myself (and I didn't fall through).

How to reproduce:

1. Use /fill to create a 1-block thick platform of stone (I used 35x35)
2. /summon a Zombie (or Giant)
3. Watch as it falls THROUGH the platform into the void

Expected Result:

1. The zombie (or Giant) would LAND ON the stone platform and walk around

Actual Result:

1. The Zombie (or Giant) PHASED THROUGH the stone platform, took damage as if it was stuck in gravel or sand, and fell through into the void.

I'm pretty sure it's not supposed to do that.

UPDATE: After about 200 Zombies and Giants falling into the void, all of a sudden it stopped happening. I don't know why.