IIRC, on an iPhone, Pushover requires some additional configuration for the Emergency [E] priority to work properly. Perhaps that is needed on Android as well?
I personally use notifications such as the one below and they work fine (I am using @ogiewon's enhanced Pushover driver). Make sure you construct the notification using instructions in the documentation that accompanies @ogiewon's driver - if that's the driver you are using.
This is how an emergency ([E]) notification should be constructed as:
Works fine for me, using my Pushover Driver. I haven't used Hubitat's built-in driver in a quite some time. Thus, I cannot say whether or not it properly handles the Emergency [E] priority.
Make sure you have values set properly for the following two User Preferences on the Pushover Device, as these are needed for the [E] priority messages, if not embedded into the message as shown by @aaiyar's example.