1.16.4-1.13.9
In a chunk with a load level of 32, but without a player nearby, the Mineralis ritual does not convert stone to ore blocks. As soon as a player comes close, the conversion starts again.

There must be some special circumstances causing this. I am on the same version of Astral and the Mineralis ritual is working even when there are no players online.

I guess it needs a chunk load level of 31 or lower to function. The question is if this would be considered a bug or not.

I don't think that's a bug, because of the chunk level ticket changes starting in 1.14. Chunk load 32 means game ticking occurs so things like redstone circuits keep working, but not for entities. This should also mean that block changes aren't going to trigger.

Water can still transform into stone in combination with lava in chunks with a load level of 32, which is somewhat similar. I'm also not sure if this would be considered a bug or not.

Hard to say? I'm checking if entities can tick inside a chunk this is attempting to convert stone in; so level has to be <=31