In my personal opinion, getting UNDER a robot while playing defense is far less of an issue than getting ON TOP OF/INSIDE one, as long as you seek to reverse the situation immediately. The underside of a robot is pretty resilient, and the defending robot is assuming greater risk letting the defended bot get on top of them, where the more sensitive components are.
Bumpers and CG will shift; sometimes you’ll get under a bot - just reverse course IMMEDIATELY and do not exacerbate, and you’ll be fine.
Now if you end up getting on TOP of another while playing D, you should be opening yourself up to MUCH more scrutiny and immediate penalty, because now your drive wheels become weapons of death and dismemberment. Lead with your heavy end, defenders.
Any and ALL cases of frame incursion by an aggressor with a non-drivetrain appendage should be called IMMEDIATELY, as such appendages tend to rip wires, tubing, and do other various and sundry carnage in short order. Inspect your robots at match end, show the damage immediately, and camp in the question box until the referees get their act straight with regards to any such transgressors, whether they be malicious or ignorant.