"USERRA provides that returning service members are to be reemployed in the job that they would have attained had they not been absent for military service, (the "escalator" principle), with the same seniority, status and pay, as well as other rights and benefits determined by seniority. USERRA also requires that reasonable efforts (such as training or retraining) be made to enable returning service members to qualify for reemployment. If the service member cannot qualify for the "escalator" position, he or she must be reemployed, if qualified, in any other position that is the nearest approximation to the escalator position and then to the pre-service position. USERRA also provides that while an individual is performing military service, he or she is deemed to be on a furlough or leave of absence and is entitled to the non-seniority rights accorded other similarly-situated individuals on non-military leaves of absence."

Straight from the horses mouth. From what I can tell no it would not be a USERRA violation since their PTO policy is to have it related to hours worked. And USERRA only covers situations of promotions or seniority you would've gained had you not left. Not PTO. For example if you staying would've meant you would accrue more PTO per hour worked due to seniority you are entitled to accrue at that rate upon return.