seen this behaviour on my own Android 10 devices (up to v1.15) with Signal installed through AppLounge and didn’t investigate yet - it’s not specific to Signal. Only Signal complains in-App to need an update “or else”.
I need to invoke AppLounge → Update Apps manually let AppLounge check for updates and download & apply. There seems to be no open bug on this in the backlog, all checkmarks on in settings to always update/install. I ran into a similar issue at AppLounge Updates stuck? - #9 by tcecyk - but that got solved. Is there a more current thread on this behaviour?