EDIT: It actually has a limit that is 512. my bad. Now the problem is that things are overflowing to the rest of the slots. Is it always a thing? Is it a good idea to keep away items from going into the empty, unfiltered slots if the network is full?

Using build 4528738
When the network is full, items transferred into the ME interface go into its internal item buffer, but there's no limit on the stack size.

nvm that's actually not a big deal