Liquid bits, both water and lava, seem to cause allocation rates in excess of 2.5GB/s on the client when a player stands in them, or in the case of the CABIN modpack, something else is interacting with them. (a create water wheel seem to do the trick) Does not seem to happen with bits of solid blocks.
When not interacted with, allocation rates go down to baseline levels. While this doesn't necessarily cause problems, it does put immense pressure on clients with constrained memory and weaker CPUs that need to get through this large amount of garbage collection. Frequent and large garbage collection spikes are a symptom of this. Reproducible for every player on our server.

Here's a small demonstration, not quite reaching the same heights but you get the point:

Game log: https://mclo.gs/AlrTdtR

Minecraft 1.20.1, Forge 47.4.0