When using a constraint to disable collision between the constrained bodies, collisions are not ignored if the constraint is part of a streamed in sub level that does not block on load. This also occurs for AlwaysLoaded sub levels.
Create an actor BP with two colliding simulating bodies and a constraint between them set to disable collision.
Place an instance in a parent level and sub level.
Set the sub level streaming method to Blueprint, and load the level on begin play.
Result: Collision between the overlapping bodies is ignored.
Set the sub level streaming method to AlwaysLoaded and play.
Result: Collisions between the overlapping bodies of the sub level instance are not ignored.
There's no existing public thread on this issue, so head over to Questions & Answers just mention UE-79873 in the post.