If the printer is old enough, SON may be enabling a feature called 802.11r (Fast BSS Transition) which allows for devices to quickly roam between wireless radios as well as access points (FiOS Network extenders) without much of an interruption. Older devices tend to break when 802.11r is enabled, and will either claim the network is a WPA2-Enterprise network, or be unable to authenticate despite showing the correct encryption type. Normally, you would want the option to disable 802.11r while leaving the rest of SON enabled.
As a workaround, consider dropping the transmit power of your 2.4Ghz wireless radio, say to 50%, while keeping 5GHz on high. This will help convince devices to join 5Ghz more often when the network name on each radio is the same. Additionally, if you have a FiOS network extender, manually set it to operate on different channels from the FiOS Router. Channels 1, 6, and 11 are non-overlapping for 2.4Ghz. On 5Ghz, note that Channels are grouped together in series of four, for a total of 6 non-overlapping channels (as the routers run in VHT80 mode). So channels 36-48 are one, 52-64 are one, 100-112 are one, 116-128 are one, 132-144 are one, and 148-161 are one. Make sure at least ONE router or access point is using channel 36-48 or 149-161, as not all devices support the other channels (which are known as DFS channels). Rokus, Smart TVs, and other "IoT" devices like smart cameras and doorbells tend to be the biggest culprits, so prioritize non-DFS near those non-supporting higher bandwidth devices.
For AP roaming, note that many devices start to scan for access points starting around -75dB signal strength. You can use Wi-Fi diagnostics or Stumbling tools to figure out your signal strength. Adjust power levels and placement of your network equipment to reach your desired overlap/roaming setup without SON.