IIRC the reason most caps used to be unrecyclable (many still are) is that they had a liner in them made of a different material. Because such caps were composite materials (using different types of plastic for the liner and the cap), they would make an impure product if recycled. The same problem applies if the cap and bottle are different types of plastic, which used to be more common.

My gf told me that people are suspicious of me because I don’t use regular social media :(