So I am not an expert in any regard about adhesives...
Might it be that both blocks were exposed to higher humidity? It appears that 69's gum became tacky to a degree having pulled off a bit of the next sheet face color, or the face ink was absorbed a bit by the gum? I can't tell by the photo if the gum in simply discolored or there's surface material there.
As for 66's darker paper color that doesn't show on the photo - that might have occurred when some of the gum dissolved into the paper. So I theorize.
At times like these I sure wish we had expert printers available to talk about such matters. In another thread we were discussing stamp and catalog reference colors. Don't those folks that print stamps collect them and join such boards such as this?
Waazwi - IMHO Your humble opinion may differ. Do not make more than two humble opinions a day. If your humble opinion always differs from everyone else's, please see your doctor. Results may vary.
Magguss - unfortunately there isn't much to be done with these. They're not an expensive PB and are two of several ways collectors years ago thought they were protecting their purchases when, indeed, over time, the philatelic value decreased.
I've seen many binders full of full sheets and PB's where the gum has become a "liquid gloss" or a dull finish similar to yours, or tacky to the touch, partially eaten by bugs, mildewed, absorbing the color of the craft type paper they were mounted on and so on.