It has happened as recently as last month with a relative of mine. On that phone, the messages would get delivered only when Signal was manually opened; if Signal was not the foreground application, messages for it would not get delivered (but WhatsApp delivered messages correctly on the same phone). It's as if the Signal server had lost the association between the Signal account and the GCM token used to deliver the notifications.

I think it used to be the case back in 2015 or so, I believe they meanwhile fixed the issue. For sure it hasn't happened to me for a couple of years now.