Category for a task blueprint(BTTask_BuleprintBase) does not appear in behavior tree's context menu after reopening a project, if the task is not placed in the behavior tree. User needs to open the task blueprint or place the task node in the tree to see the category.
I could repro the behavior in 4.14 and 4.15 Preview4.
1. Create a new task blueprint using BTTask_BuleprintBase
2. Set the new blueprint's Blueprint category(for example, "TestTask") then compile
3. Create a new behavior tree and from the context menu, see the category set in 2. is shown under Tasks.
4. Save all then close the project
5. Reopen the project
6. Open the behavior tree and check the context menu - the category is now shown.(the task itself is shown)
7. Category reappears after opening the task blueprint or placing the task node in BT.
There's no existing public thread on this issue, so head over to Questions & Answers just mention UE-41799 in the post.