I have seen that one or two times when using cast acrylic, extruded is far worse. (If the rastered lettering is not snow white, it is extruded.)
When the covering is lifting from vacuum or air assist, the heat going under the coating gets trapped and causes these types of marks. If you have it on the painted side also it sounds like heat affecting the acrylic. Again extruded is much worse than cast.
On acrylic I would not recommend two low power passes, the acrylic expands enough to make the second cut in a different place than the first cut.
Check to be sure that the vacuum line is totally free of bends and flats that might be reducing the suck. Be sure all metal pipe connections are tight and covered with tape to stop air from leaking in and reducing the vacuum. A faster changeover of air might solve the problem.
Philadelphia, PA (Really Bensalem)
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