CORRECTION: Both Monster and the Ultra Pro binders are indeed acid free.
THAT BEING SAID, the problem that I describe is REAL, but acid is not the cause. Many people have reported that storing unsleeved cards in binders such as these for the LONG term (like, 8 months or more) without moving or touching them can lead to the cards becoming stuck to the black backing.
I incorrectly assumed this issue meant the backing was not acid free. Although I have now confirmed the backs are acid free, the issue is still real.
This is not a major problem, and only affects a few specific users, but it is indeed a real issue none the less. I always advise my viewers to sleeve any cards that they are storing long term in portfolios, and to always sleeve all cards of real value.
This is not unique to Ultra Pro and may be an issue relevant to all portfolios. However, because it requires 8 months or more to create these conditions, it is very hard to test for.