edit: crap, I'm sorry. I didn't do the title right and I can't change it or delete the msg and start over. sorry :(
[1.2.3] (an attempt was made)
I've done this setup many times previously
Setup stuff in a factory to make something, inside, pick one of the lanes and put a requestor chest, choose what I want outside factory, put a storage chest on outside and it balances out automatically

I did this with the krastorio production and utility tech cards and suddenly it has a bizzarre behavior.
requestor gets a stack, sends WHOLE stack outside where storage gets the stack. 1 second later, requestor gets another stack and everything that was outside, sends EVERYTHING outside. so every second it gains a stack of cards and neither side keeps them for more than a second a piece

it's ONLY doing this with these tech cards. I've got multiple setups with zero issues in my current save.
factorio 1.1.82
factorissimo-2-notnotmelon_1.2.3
Krastorio2_1.3.19
other mods ofc but these are the important ones.

I noticed a couple other issues that point the problem is stuff with metadata. I wish the mod portal allowed you to delete posts

Yep, its intended. I don't want to modify stacks with custom metadata because it leads to duplication glitches or item deletion.

For example in the original factorissimo, sending power armor through a chest connection will wipe all contained equipment.
Or sending science packs would reset the durability to full, causing a dupe.