I don’t know if the University products envelopes are a different material, but the old glassine envelopes were very damaging, especially as they aged. I have seen photographs with a damaged brown line across them, where the seam of the glassine ran – acid products had leached out of the cut edge of the glassine paper at a higher concentration.
If you’re having to handle the photographs for scanning, I wonder whether polyester enclosures wouldn’t be better anyway – they provide physical protection as well being chemically more suitable. My apologies in advance if the UP glassine is a different substance to the material used in the past. Has it passed the PAT test ? https://www.imagepermanenceinstitute.org/testing/pat