I was able to make rennet out of thistles and water in the brew kettle, but I am unable to put it in the cheese vat to make cheese.

The cheese vat is over a fire block
I have tried putting the rennet in first, cannot do.
I have tried putting milk in first, and tried adding rennet after each bucket, cannot do.
I have tried putting in 5 buckets of milk and starter culture, and then rennet, cannot do.

I have also tried this without a fire block, wondering if I had to heat it up after filling it: no joy.

Is this a bug, or has the process changed?

You can try to use rennet bottle instead of bucket. This should work. If not, then it's possible a bug.

Aha! Yes, that did it! Thank you. (I had to put 5 buckets of milk in first, then the bottle of rennet, then I put in the starter culture)