Revoked certificate?

I've been getting the following notifications from Bitdefender recently:

"firefox.exe attempted to establish a connection relying on a revoked certificate to dns.nextdns.io. We blocked the connection to keep your data safe since revoked certificates are usually fraudulently obtained or used."

On the surface, i don't seem to be affected in my usual internet browsing. Other than these notices (upto hundreds a day) , I don't notice anything annoying yet. NextDNS still seem to be logging and blocking as needed.

Should i be worried? Or is this just something that hopefully NextDNS is already aware of and is in the process of issuing valid certificate?