KRISS magazine extensions are designed and intended for use in the G21. You installed a KRISS mag extension in a G20 magazine and it failed. That has a lot to do with the follower design.
I talked with KRISS regarding this subject. I was informed they intend to introduce a 10mm kit late this year. If you do not want to wait for the kit you will need to work the bugs out of your current follower. Look at the difference between the G21 follower and the G20 follower. Try to modify the G20 follower to resemble the G21 follower and you will have a good start.