We are experiencing a similar issue as these two questions:
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If you have a BP with a static mobility default root, sometimes it will just reset to mobile in it’s child blueprints. And when you diff the blueprint, the engine says there is no difference, even though mobility was reset. Even if we fix it and resave, after restarting the editor, it has reverted itself back. Even setting the mobility to static on construction will not solve the issue. The only way to temporarily solve this is to manually recreate the blueprints and delete-replace the old ones. Incredibly hard to repro in a blank project I assume, but I see others have the same issue.