I suspect the bubbles are water that leaches all the way through the tank material but I honestly dont know definitively. Why dont you prick a bubble and see if any fluid comes out.
If the paint bubbles and you wrap over the paint then obviously you will have the wrap bubbling as well. You cant wrap over an unpainted tank because the only way to get the paint off a plastic tank in reality is to media blast them and thats what I've had to do in the past. Then the tank surface is too rough to wrap.
My main question with the wrap and I've seen a bit of it done on youtube and its amazing stuff and the guys are skilled is, is it fuel proof from the point of view of spillage during refueling and the inevitable constant exposure to slight fuel vapors around the filler cap fitting etc.
Ciao