I see the same. Indeed, whatever the ‘Start Upgrading’ button turns into when the upgrade starts - I think it used to change into some kind of Cancel button - I notice it pretty quickly went back to ‘Start Upgrading’ while the upgrade was still in progress, and remained there even after the upgrade was successfully completed. I’m running Chrome Version 86.0.4240.80 (Official Build) (x86_64) on a Mac.
I saw this on a previous upgrade. The present upgrade is to 2.6.0.beta4 from 2.6.0.beta3. I think the immediately previous upgrade required an app rebuild.
Edit: on my other forum, I just saw the same, and took some screenshots. The button flipped back while the log was still showing one of the many Waiting for Unicorn to reload messages at the beginning. (I speculate that the flip is based on time having elapsed, not to do with the progress of the upgrade. Edit: 60 seconds from button press to reversion. Whereas the progress bar first moved at about 90 seconds.) See below:
