Never get any notification when phone was switched off

Gotify works perfectly and I am verry happy with it - thanks to the developer! There is just one niggling problem that I just can't solve no matter what.

I send a push message with Curl to my gotify server and the gotify app on my smartphone notifies me immediately with all bells and whistles. So far so good. But when my phone is switched off completely during this command (and I switch it back on later again) it just won't notify me at all: No red dot on the app, no message, no sound - nothing. It just stays silent no matter how long I wait for something to happen. When I open the app the new notification is there - it just never notified me in the first place.

Is this an issue with the server, the app, a setting in Android or is there some kind of misunderstanding on my part here?