On my first hub it said there was NOT an upgrade when I did the "check for updates" . After I rebooted the hub it found the update file.
On my second hub it would freeze during the download and not download the full file. After a reboot it downloaded the update just fine.
On my third hub everything was fine, saw the update, applied it, etc.
My suggestion (which I should have done in the first place, to be honest) is to reboot before checking for/installing the update
(all hubs are on the same subnet and have minimal CPU loading btw) I'm in a rural area and controlling lights on multiple buildings is beyond the range of one zwave network.
For the last few months, I have been following this same procedure (i.e. reboot the hub before attempting a firmware upgrade.). Doing so has eliminated all issues that I had experienced previously, as well as the reboot creates another local database backup on the hub which is nice to have in case I need to revert.
I had no problems with backup thus far without any sort of reboot, but given your experiences, I don't see harm in rebooting. I'm not sure what consequences @ogiewon and probably others have seen other than the inconvinience weirdness @eric4 described, but I'll assume your reboot recommendation helps and avoiding is worth the small delay.