From official documentation https://argocd-image-updater.readthedocs.io/en/stable/basics/update-strategies/

argocd-image-updater.argoproj.io/myimage.allow-tags: regexp:^[0-9a-f]{7}$

That means it must start with regexp:…

Also take a look at regexp syntax, cause ^ means input beginning, and $ input ending, with this changes, your annotation would be:

argocd-image-updater.argoproj.io/rtm.allow-tags: regexp:.*SNAPSHOT.*

Can you provide update strategy? It will helps to fully understand the situation.