I believe that some manufacturers impose a “whitelist” sort of thing for apps running in a background. I know a case of a Motorola and an ASUS phone that basically alllowed only the popular apps like Whatsapp and Facebook to run in background and Signal was terminated almost instantly when put into back. Samsung for example has a tool to detect if the app is “noisy” and if it shows too many notifications then the system will stop showing them for a while. They probably have an allowlist themselves because I haven’t heard about this issue on Messenger or Instagram. How it works internally is impossible to say since we cannot see the code.

I was working as an Android developer couple of years ago and we had to fix many issues with push notifications and these were the team’s findings. It may be no longer the case and I believe it’s time Signal started showing up on these allowlists but who knows