I never saw this happen on my Levetta. When backflushing, how long do you run the brew cycle for? If its longer than 6 seconds then maybe you are doing it too long.
My routine was add about 1/4 tspn backflush detergent to the blind. Add small amount of hotwater to dissolve the detergent. Lock into grouphead. Lift lever until you hear the pump change tone (5 - 6 seconds. Disengage brew lever. Wait for about 20 seconds for the detergent to work then run two or three more short cycles (on for 5 seconds, wait for 5 seconds). Then I rinse the blind filter and replace it in the grouphead for cleanwater backflush - about 4 or 5 cycles running 5 seconds on, 5 seconds off.