Hey Andy, I think you have auto-normalise turned on.
This can cause an issue if there is only 1 vertex group for it to normalise, therefore it adding it to the active group before it’s adjusting the weights. It’s trying to subtract the weights down to zero but the auto-normalise is recalculating things back up to 1.0
This is one of the quirks with auto-normalising with only 1 group.
If you have at least 2 groups for it to auto-normalise. it will work as you expect. (or if you disable auto-normalise it will work as well)
