This is not a bug, they are unloaded. This is working as intended
I can confirm, boats do not go the speed of ice when buttons are placed on top. This is both a bug, and now a parity issue with java. Boats are supposed to travel on ice with buttons at the same speed they would without. But this isnt happening. I have linked a video clip in the file section to see.
This is an intended feature. Hoppers only pick up blocks from partial blocks. Hopper minecarts can pick up from full blocks.
It looks like your world corrupted
Confirmed for 1.16.1. This also affects flying on the underside as the player takes damage.
Video: https://youtu.be/fyHQ2yRcH9Y