hello! I create custom mod pack with Create:Molten Metals! But if I put molten zinc + molten copper -> (I get hotted blaze burner) I don't get molten brass ):
Can you help me with this?

This bug also occurs for me. I'm on version 1.20.1-0.1.4

I'm having the same issue on 1.20.1 0.1.4 as well, so it's not an isolated issue it appears.

I've bumped into this too and this might be a Create limitation?

My guess is only items and liquid that have been inserted into basins can be used as recipe ingredients. Those that were the result of a crafting recipe are "output" items / liquids. In the case of liquids you have to pump them out and back in, into another basin or the same one.