Over the past couple of days I've seen a few occurrences of people on other servers verifying a new device, but my server not receiving the resulting signature, thus leaving them with a red shield from my pov. I've also seen people saying they were seeing others with a red shield whereas I would see them with a green one.

@bwindels had a look at my Riot logs upon failing to see one of @anoadragon453's devices as verified, and could see that it was indeed missing a signature, with the signatures on Andrew's device being:

"signatures": {
  "@andrewm:amorgan.xyz": {
    "ed25519:ZYMXYYQZTP": "aBAnNnO3Qzz8xMLXtSjnwDummc4F4vewygUBvdRKJWd2/326Se4knYkJkp/TKa0sXjeGKIUko8JXNorN2m9TDw",
    "ed25519:QjSD8srN17RiDzIBgzVbncj+NMdvDHRY4N2b8w+oq9Y": "FV5bCKEdp1q9SFvy7MzUgTj3SzAeHwuK4vvtgnIp/WFxdnYQ4XL2C64dW28oJsK4yJ95NkuFcL9uCTPaDKPoAw"
  }
}

and the ones my Riot would see for that same device being:

"signatures": {
  "@andrewm:amorgan.xyz": {
    "ed25519:ZYMXYYQZTP": "aBAnNnO3Qzz8xMLXtSjnwDummc4F4vewygUBvdRKJWd2/326Se4knYkJkp/TKa0sXjeGKIUko8JXNorN2m9TDw"
  }
}

Looking at the device_lists_remote_cache on my homeserver's database, I can see that it's indeed missing the signature from Andrew's self-signing key (ed25519:QjSD8srN17RiDzIBgzVbncj+NMdvDHRY4N2b8w+oq9Y).

https://github.com/matrix-org/riot-web-rageshakes/issues/2740#issuecomment-623992322 provides more info about that specific occurrence.