As the title states, I seem to have encountered a bug wherein, any time I'm at peace with a Lumeris faction in ES2, they send me a constant stream of diplomatic threats (over and over again even in the same turn). The bug occurs both when I am at peace due to a treaty I myself forged and when an alliance I am in elicits a treaty. I have replicated this bug in three games (all three of the games I've played so far).

This appears to be the same bug as this one and this one. It's supposed to be fixed in patch 1.0.11 currently in preview.

Yep this problem should be fixed in the patch preview 1.0.11 :)