It seems like the Logistics Teleporter will only trigger its "teleport when full" condition if it's full of only stacks of the same item. If it happens to fill up with multiple items, then the "teleport when full" condition will never trigger. Is this intentional? Can this be made optional?

As it stands, I was able to get around this limitation with some circuit logic. (each + stack_size - 1)/stack size = total_stacks, then if total_stacks >=48, send planet signal. However, that rather undermines the "teleport when full" condition when I'm having to make my own alternate "teleport when full" condition via circuits.

I filled a teleporter with full stacks of multiple different types of items (with different maximum stack sizes, too) and it activated as expected. Please post screenshots of your teleporters including the inventory when it should be activating, and the current settings, and I can try to figure out what is going wrong.

Actually, I figured out what was going on, and it was user error. The teleporter was "filling up", but not all stacks were at max. As a result, it wasn't technically "full", but rather just appeared so on cursory examination. That was entirely my own fault. Please disregard. My sincerest apologies.

Not a problem, I'm glad it's working!

Trains have the same problem, if they're being filled by a mixed source they can get in a state where the inserter(s) that are filling it are all holding items that won't go in, but the game does not think it's "full". I think the real fix would be for the game to implement a concept of mostly full. Something is mostly full when all inserters facing it can't insert. Or, to reduce the computations I would even accept a definition that there are no empty stacks.

In this case, you need to make sure every source feeds only one item and if you're divving up a belt each type needs a buffer of at least one stack.

What I did for trains was to simply have an extra loader filtering OUT of the train being loaded. That way, any junk left over on the belts simply filters into and back out of the train. Unfortunately, that's not something I can do with Teleporters without making it super-complicated, since I'm relying on Project Cybersyn to generate train "requests". One teleporter per item IS an option, granted, but a bit overkill I think.

My solution - as posted in the OP - was to simply sidestep the "full" check on the teleporter itself with a bit of circuit logic that forces a teleport when all slots are filled. Means I tend to teleport with somewhat fewer than maxiumum items reducing throughput, but not by enough to really notice.