I have just tested on Android 5.1 and it works fine. It takes a few seconds to rotate, but it does rotate. Probably your device is just slow and you have to wait a bit more.
I just checked it again, and I think we both have a part of reason
ManagerApp rotates and expands when we first access the reports screen.
Now we go into report settings, select a device and confirm.
Then ManagerApp only rotates the screen, but does not expand it.
Looks like it's another bug in ExtJS library.
Expand to screen size, after turning the device (Android V 5.1)
Traccar Manager App:
Browser Google Chrome: