I'm saying it's possible, and even more convoluted than you think.
Could be an instance where it got currupted on one device, and another deivce just happened to pick up the bad lookup for safe keeping, then repropagated it the next time any other device tried to go there.
If that's the case:
1. I'd switch everything to point to the modem DNS (automatic).
2. Then power everything down with the modem being last.
3. Then, power everything back up, modem being first to own the master DNS, with everything else following afterwards.
4. If you want to change the DNS on each device to point to something else after that do so.
This way you be assured that any corrupt entry is completely expunged from your local network.
Business to attend to and a business trip so will have to try that when I get back.