I may have just answered my own question about my Server 2022 -> 2025 update.
Both my live and test servers have Hyper-V and have at least one virtual machine. On my test server I'd taken the time to stop the VMs on there first, and that's what let it continue.
On the live server, I hadn't stopped/saved the VM
When looking closer at the XML output of ScanResult.xml, it clearly says right there:
<Plugin Id="HyperVComplCheck.dll_HyperVUpgradeComplianceCheck">
<CompatibilityInfo BlockingType="Hard"/>
<Message Text="Setup has detected one or more running virtual machines. Please save or shut down all virtual machines, and then restart the installation."/>
</Plugin>
I didn't see it at first because all of that part of the file is one long line, so I had to look closer to see that message. I'm still annoyed that the setup program isn't showing any text to tell me that's why I can't continue.
Anyway, hopefully this helps anyone who might see this. Since Win10/Win11 can also run Hyper-V, I suppose it's possible that even one of those could encounter this same issue.