I wouldn’t be surprised if Avast hadn’t released it, we have seen this before, a new beta released followed a day later being released as stable update (and worse still with the same build number, causing further confusion). Though I haven’t seen it in my About Avast or Update UI screens as I have Program updates set to Manual.
That would be strange, if you are on the beta stream (e.g. a beta version installed) then you should be offered beta stream updates, not the regular update stream. Since your beta build has a higher build number it shouldn’t go backwards to a lower regular update stream.
Effectively you should have got beta build 18.1.2325 if you had 18.1.2324.
This might happen when servers.def gets corrupted or otherwise is not available. Avast then downloads new servers.def from hardcoded fallback URL which points to release one. That in fact switches update channel to release one.
For couple of years we publish release builds to beta channel and then when no new critical issues are found (or we have micro updates for them) we publish them to release channel. Exactly same build, exactly same binaries, exactly same build numbers. This way we are publishing to release channel only what we have tested and you have helped us to validate.
Keeping same build for BETA and RELEASE at that point also kinda makes sense so BETA users after they finish the BETA cycle actually have the RELEASE version till new BETA is actually posted on their channel again. This way they are cycling beta and release versions, making testing in a way more extensive than if they’d constantly be rocking just beta builds that are always different than the public versions.