I think this particular issue is likely just a regional distributor matter at this point. Especially with Seapora. They're Canadian and don't import as much glass from India and China as the larger brands like Marineland and All Glass.
The smaller tanks (under 100gal) I use the most are Seapora (formerly Deep Blue Professional) and almost always have to have the distributor for my region bring them in. Have had to do it twice since the pandemic started but I only had to wait a few days each time. There have been a few Seapora sales the past few months in the US & CA because of overstock, even. That's something that's rarely happened. A majority of their glass is manufactured in North America, so it's possible they are not impacted like the others. Their small high-clarity tanks come from the OEMs I'll mention below.
If you have other shops in your area you can work with to try to get the tanks you want, it'd be worthwhile to reach out to them.
There is a global glass shortage, due to sand shortage problems, but that's mostly with bottle manufacturers and scientific (borosilicate) suppliers surge producing vaccination vials. There hasn't yet been a standard quality aquarium glass shortage. Emphasis on yet. The only thing that's really been hit is high-clarity glass. It's been in higher demand because more people are staying home and starting up tanks. Lead times have slowed because of the length of time it takes to get a container from the OEMs in India and China to North America. There are only a couple high-clarity tank OEMs that produce for the current major brands like ADA and UNS, so that's part of the issue. Aquatop and the smaller/lesser known labels buy from the same source(s) but they don't move as much product and likely aren't impacted.
Another reason I don't think the glass shortage has hit the overall tank industry yet: Petco & PetSmart have had several dollar-per-gallon sales recently. I've bought a ton of those loss leader tanks just to have them on-hand the past few months.
edit: words are hard sometimes. ugh.