Found on Forgecraft. The assembler does not seem to properly handle which positions can have a pipe attached. This pipe can input.

Additionally, JEI integration appears to be borked, it's showing the message of "All pattern slots are filled" even though they are all empty.

And it doesn't craft concrete. Probably an issue with the fluid ingredients.

This looks like a mod interaction of some sort to me, none of the 3 issues is present in either my 1.14 or my 1.16 dev environment.

That makes it worse. With the size of the FC pack, this will be a nightmare to debug.