Shimano bolts; oversized hex driver holes? Planning for failure?
Well, it's no just you, my ST-R9120 bleed srew accepts 2mm (as stated in manual) very loosely too, 2.5mm is too thick, there's even a whole thread on WW dedicated to the issue, apparently those screws are/were of very poor quality, this was not the case with ST-RS685. Some of proposed "solutions" are:


Hammer T10 in. Now you have a T10 screw, but this still might not help or even damage the threads.
Or file a flat screwdriver slot and use it instead.
Contact Shimano for a warranty.
Buy replacement screw(s) (Y0C698030).
Try to push hex key further, some people stripped the slot by not pushing the key deep enough.
File a 2.5mm hex key down so it fits tightly.


Update: among three sets of hex wrenches I have access to, one had 2.5mm  tight enough not to strip the bolt. Both bolts unscrewed with a slight click. So, #6 worked for me.