Strange behaviour: can't connect with MAC Address Filtering enabled

So I'm minding my own business, my router works fine. It's a Linksys E4200v1 running latest Tomato.

All of a sudden no WIFI devices will connect. I tested turning off various services one at a time until I singled out "MAC Address Filter" (a.k.a. "Wireless Filter" on Tomato UI)

I've been running the router with MAC Address Filtering enabled ever since I installed the router I think almost 2 years ago.

So now, all of a sudden, none of my WIFI devices will connect if MAC Address Filter is enabled.

If I disable it they all connect fine.

Without getting into a debate about whether or not using MAC address filtering is an effective security measure, I'd just like to understand how (what I'll describe as) the circuitry that controls MAC Address Filtering can "break" but the router can otherwise function normally. It seems to me that whatever bit of the code that deals with the incoming traffic couldn't 'rupture' just the bit that handles MAC Address Filtering but otherwise working normally.

I don't like buying new hardware the first time something odd happens, but this is the kind of problem that makes me think that the router is seriously impaired (a memory chip failure?) but I can't even imagine how just the "Mac Address Filter" could all of a sudden be a problem when it's worked fine all this time, and the router works fine otherwise.

Is there any way to do a deep diagnostic on the device?

 

It's not handled by hardware in any way so it can't be a problem with the router itself - it has to be a software issue with Tomato.
I know it a pain to do but if you want to test it back up your Tomato config, flash official Linksys firmware and check if MAC filtering works there correctly.

 

Yeah... I was going to try doing a hard reset of the router (but without loading the Linksys firmware... just wipe Tomato back to defaults and reenter the data), just in case something like the onboard NVM got corrupted somehow... but when I look at the MAC address table the addresses are there. I'll try that first, then try the Linksys firmware, and report back with my findings.

Thanks