Stationary light in a blueprint gets displayed with red 'X' in Blueprint Editor viewport after opening the blueprint for the second time.
1. Create a new Blueprint Actor
2. Add a point light component to the actor, make the light the root, then change it to stationary mobility
3. Compile and save the blueprint
4. Close Blueprint Editor then reopen the actor
5. Switch to the Blueprint editor's viewport to see the light is now displayed with Red 'X'
There's no existing public thread on this issue, so head over to Questions & Answers just mention UE-49555 in the post.