This is happening on my project as well. There are dozens of blueprints that switch to default parameters. I worked around some of it by overriding the parameters with literal values on BeginPlay, but that doesn’t help at all for blueprints that are either never spawned or come into play via c++. Also it’s a mess.
My problem does not involve any blueprint actors placed in levels. Everything is spawned at runtime, It may involve creating a new child of a parent, which then triggers some of the other children of it losing their values, though that is speculation at this point.
As our game is very content driven and it’ll only get worse as the number of blueprints continues to mount, it’d be great to have an update here.
EDIT:
This is with version 4.13