Can you elaborate on what changes you have made and how? I have just tested this with 7.9.1 by removing a component from the reuse block and verifying the block, I didn't change the reuse block symbol and it was flagged in DxDesigner as needing update. Two things to check, try opening and closing the project to see if this triggers the notification, and ensure the 'Flag out-of-date reuse blocks' option is enabled in your advanced settings, it is separate to 'Flag out-of-date symbols'.
The problem may be in the order something was done if you could try the following to see if you get the same result that would be appreciated.
Make a componet delete in the block schematic and after that is done make a variant change in the variant manger before verifying the block again.