Why are my 5ghz and 2.4ghz SSIDs suddenly fighting with each other?

For a long time I've had an XFinity cable modem and my 2.4 and 5 ghz SSIDs have always had the same name.  About a week ago out of nowhere my laptops and phones could no longer connect for more than a few seconds at a time.  The only way to get wifi access was to connect via a LinkSys extender I had purchased a while back to increase my overall coverage range.  I opened ticket CR914371636 with Xfinity support and after about 90 minutes of troubleshooting I was told the modem was failing and I needed to exchange exchange it.  I did that, and the problem did *not* go away.  Noticing that a couple of older devices that only support 2.4 ghz still worked fine connecting directly to the SSID, I changed the name of the 2.4 ghz SSID to something different, and now all of a sudden everything works fine.

My question is - what changed?  I had the same SSIDs running on the same model modem literally for years, and a few days ago they just started fighting with each other.  Anyone else see this issue?  Is there some other solution rather than my workaround?

Thanks in advance,

-Mat