Looks and sounds like you’ve narrowed the problem down. I can’t immediately recall having come across this with Shimano shifters, rather the opposite that the ratcheting mechanism no longer engages or skips clicks. Would be interested in what else you find.
The tension spring is pushing that lobe down further than it can go on the 105 unit which is why it's fouling. I need to get another unit to compare it.
Looks and sounds like you’ve narrowed the problem down. I can’t immediately recall having come across this with Shimano shifters, rather the opposite that the ratcheting mechanism no longer engages or skips clicks. Would be interested in what else you find.