Cover the front with electrical tape, old coin op game license tag, logo decal etc. Otherwise replace the glass.

Clear packing tape

99% of me says don't even touch that - it will become worse.

The other 1% says spray the back with clear acrylic, it won't "fix" the area but will help to arrest the process of decay that is almost inevitable.

Shot of Triple Thick + small piece of Saran Wrap + big fat book = flat bubble.

Unless you have done it before and are comfortable with it I would leave it alone...That bubble will not adhere to the glass unless Triple Thick is sprayed under it...or between the artwork and the glass, then spray more TT on top, and then put a piece of Saran Wrap over it and something heavy to lay on it for a couple days...Otherwise it is not going to stick it back down, it will crackle it and end up showing light through the crackles. Then it will need touch ups etc.

Phoebe

The Triple Thick / Saran Wrap method is very advanced. I wouldn't recommend it to anyone who is asking this question. That's the sort of thing someone learns on trash backglasses after having already successfully coated several backglasses. Someone new to the method is far more likely to destroy the area than save it that way.

IMO, your choices here are leave it alone, or do a standard Triple Thick job on the whole backglass. If you Triple Thick, treat that spot no differently, and if it is fixed it is fixed. If it is not fixed, you'll scrape it out and do a touchup. Or just leave it.

I would Triple Thick the glass if there are any other areas. That one minor area, meh, leave it. But if there are others then do it. Give the damaged area a couple of super light starter shots and then start with a misty coat over the whole backglass. From there three normal coats and it's done. Be sure to mask off the scoring areas. I usually cut some cardboard of the right shape and weight it with quarters.