definitely not fingerprints, more likely some kind of chemical/oil.
Fingers can get into all sorts of chemicals and oils. The human body itself produces oils. Food products, make-up, fabric softener, skin and the list goes on can transfer chemicals and oil to fingers even if wearing linen, cloth or other hand protectors.
Those sheets don't just appear in a tube neatly rolled up. There is human involvement.
Folks working in stamp fulfillment do not always understand what they are doing. My prime example was when folks could send in requests for the incorrect Legends of the West Sheets being awarded as a lottery. If you were to win, great, if not you got your original envelope back with the request and a "sorry not a winner" letter. One knew this in advance, so it was at least an opportunity to get some nice good used postage returned to you if you affixed it to the envelope.
All the loose paper and envelopes were stacked, stapled together, folded and placed into the permit paid return envelope perhaps a size 12 or 14. Can you guess where the staple was always punched? Yes, right through the corner where your nice specifically selected stamps were affixed. That was a wonderful "Loser, thank you for your participation, FOOL" message without having to say it out loud.