[bug?] Actor relative pitch rotation stuck in 90 degrees

That’s going to depend a great deal on what you’re trying to accomplish and how. I would suggest opening a new post in Blueprint Scripting section with as many details as you can provide, and community can help with that. For example, SwiftIllusion posted a workaround that works for him above.

Unfortunately, there is no fix to this bug without major changes to engine, and that isn’t likely to happen anytime soon.