No, that should not happen.

Coincidentally I migrated a site from a hEX (RB750Gr3) to a RB5009 with LTE backup using Huawei stick, and now I also have a problem...
It may however be a different issue. The link was up for over a week, including some provider disconnects, however then suddenly the link disconnected, the LTE1 device disappeared, and the USB info does not show the stick anymore.
I tried a USB power reset but it does not solve it.

I will have to go to the site to further investigate, e.g. plug the stick into a laptop, plug another USB device in the router, etc to see what went broken.
Maybe the USB port does not work anymore, maybe it got overloaded by the power consumption of the stick?
Maybe I will also see the router crash/reboot then, but probably not as I already did the power reset.

Good day, recently I bought an RB5009UG+S+IN, updated it to the latest stable version (v7.14.3) and tested out with it a D-Link DWM-157 USB modem. When I was done with the experiments, I unplugged the modem and suddenly lost connectivity to the router after which I realized it had rebooted itself. I looked at the log and the first entry was "router rebooted without proper shutdown, probably power outage". I assume this is not normal behavior, am I right?

This happened to me too, RB5009 with a usb lte dongle. Not only disconnecting the dongle phisically but also executing an usb power reset.
This is not happening always, but sometimes it happens.

For me, after upgrading to 7.15.3 or 7.16beta it has not happened again...

For me, after upgrading to 7.15.3 or 7.16beta it has not happened again...

It happened to me with 7.15.3, and I cannot understand why. I don't think that it's a question of power surge because I use a usb isolator and the dongle gets power from a usb power injector, so the mikrotik usb is electrically isolated from both data e power lanes from the usb dongle. It happened first without isolation so I tried to isolate the usb dongle but the problem is not solved...