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?

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.