I have noticed some GUI bugs a while ago but was too busy to report them and thought that someone would have by now but it looks like no ones has. I can't remember how many releases back but it did display properly before, and nothing changed my my end but updated the routers fw.
The <not found> is the RE450. and for 2.4G channel it did display 2+4, these two things are and have been broke for sometime!
2l = "2 Lower channel", which is how Broadcom identifies 2 + 4. There's no bug there, this is normal.
The "not found" means that this device isn't present in the DHCP lease list, only on the arp list, so the router is unable to determine it's hostname. Therefore it says "not found", which is intended. This usually happens if the device has a static IP rather than rely on DHCP.