I build reproduction antique furniture and use different grades of shellac because they are an authentic period finish. Shellac is also easily repaired because it will always dissolve and bond with previous layers of shellac, but it's a weak finish compared to lacquer or polyurethane. Yes, as others have said, it has a shelf-life once mixed, I use flakes to be safe. Humidity can play a role during curing and I avoid applying shellac if it's humid. Shellac can also be completely non-toxic and is used as a coating in the food industry.

I also wonder if it's possible that the shellac reacted with some adhesive/solvent in the tape itself?

My question is, other than tradition, why use shellac as a preferred finish on bar tape anyway? I'd think polyurethane would be more flexible and durable in the UV light and moisture, although it might take longer to dry. Even acrylic lacquer (designed as a synthetic shellac) is more durable than shellac and dries fast too. You can even tint lacquer with dye and make it any shade you'd like. Has anyone tried polyurethane or lacquer on their bar tape?