Xenon2112,
You’ll have to excuse me if I ask some obvious stuff here…
The way you’re only seeing 3 of 20 materials in the BP setting does sound very like variable type. Are the other 17 instances? ( there’s an awful lot of ‘…material…’ possibilities in the variable types ).
As far as dropping the mats onto a simple cube in the world goes, have you tried it in a blank project?