MC-259666 doesn't seem to be fully fixed. The stall still occurs, though only when reloading such resource packs in a certain order.

To reproduce this issue, you need to have another resourcepack (that doesn't have a custom atlas) already loaded, and then load the Hoplite resource pack first before loading the MCC Island resourcepack. Presumably, because the Hoplite pack is smaller than the latter.

Steps to reproduce:

1. Load a resourcepack.
2. Load Hoplite's resourcepack.
3. Load MCC Island's resourcepack after unloading Hoplite's.
Result: Severe micro stutters would occur. Only way to fix this is to reopen the game.

Alternatively:

2. Load MCC Island's resourcepack first.
3. Load Hoplite's resourcepack after unloading MCC Island's.
Result: No micro stutters at all, no matter what order you load these resource packs afterwards.

New video demonstrating the issue: https://youtu.be/RAHWb9uYSi4