There are 2 cases that are leaking the level (one is an entity and the other is the level itself)

Here is an entity that is still kept alive, maybe nullify after render just like animatable = null on #renderToBuffer

Also, here:

This actor needs to be cleared up on level unload.

1.20.1 down

edd9f02

1.21.1 soon

Done in 1.21.1 as of 4.6.3