Normally when you fuel & de-fuel the aircraft, you are pushing fuel through it at much higher rate than will be the case when the aircraft is flying. It's not a problem if during this "fast flow" the UAT bulges or compresses a bit, as long as it can sustain normal full-power flow rate.

If you are fueling or de-fuelling extra fast though, do keep an eye on the fittings as you may actually induce a leak - I've had a UAT start leaking at one of the brass nipples when over-pressurised like this.

[edit: fix typo]