This has been the fate of ⅔ of updates via Mac App Store:
This has been going on for many months now. Through multiple updates to macOS. Same on each of 3 recent Macs (a 2018 iMac, a 2019 MacBook Pro, and a 2010 M1 MacBook Air, all running Big Sur). And on two regularly used networks. So it seems worth griping about! It’s really a thing now. MAS does 99% of the download…then declares failure…repeat 2-4 times. So much fun with a 1GB app!
Things I have tried that haven’t worked: re-launches and reboots of course … using a different network… initiating the download from different views (account, the app page). So far the only thing that has helped is removing the old version first, because then it’s just an install, not an update.
As depressing and exasperating as this bug is, it’s actually typical of my experience with macOS. For years now. #ItJustWorks 🙄