L&LD
Part of the Furniture
They're all alpha only, and the router is stable on .58.
Not trying to force you to do anything, just wanting to make my point understood.
The clients can have issues when the same ssid (maybe particularly for one that is in use for 15 years) when the router's underlying drivers and/or code changes.
I myself cannot see how this would give rise to the issues you're having with certain firmware, but it is one more variable that you can control. As mentioned, even if only tested with a new ssid's temporarily, to see if the latest firmware is stable for you as it is for so many others.