Welcome to The G, Dave. Your explanations are appreciated.
...The bad news is that because they never dry out over time the adhesive will move further into the paper making it more difficult to remove...
That is what we call migration. In agreement with what you said, migration makes it difficult to remove from fiberous materials such as paper and fabrics. Quality of the adhesive is not the issue, because even the best quality pressure sensitive adhesives could migrate. I'm no conservator, but they say the conservation treatments to deal with that are chemically invasive and not entirely effective.
...adhesives are often “tweaked” with plasticizers and tackifiers to alter the initial tack and hardness of the adhesive. These additives are prone to migration and as such we do NOT use any...
More to your point, conservators say those various additives can cause unpredictable chemical changes over time, which could make them more gummy and more migratory, or make them dry out, or otherwise weaken their bonding strength. It is good that Neschen does not use them, but some other adhesive tape makers do. Trouble is, we often do not know how to identify them.
I have never had bad results with the Filmoplast tapes I have used, but due to migration issues, I do not use
any pressure sensitive adhesives in direct contact for best-preservation framing.
Some time ago I recall hearing from a credible source about an issue of P-90 causing discoloration. I guess that could have been a unique problem caused by localized conditions, or perhaps a short-term defect in the product. Do you have any comments about that?
Neschen has provided good quality adhesive products, useful in framing, for many years. But it is important for users to realize that all products have limitations. As responsible framers, it is our job to learn about those limitations and respect them.
My speculation is that most problems of damage to framed customers' property are related to some kind of adhesive. Of all the products we use daily, adhesives are among the most mysterious and the most troublesome. When in doubt, caution is advised. And read the package instructions carefully.