Well thats odd because it all sounds right
Maybe you give this a try :
Go to wipe , Advanced Wipe:
Check Dalvik / Art Cache, System, Data, Internal Storage, Cache
After that go back to the Wipe Mainmenu
Click on Format Data (Should normaly not be needed but i would give it a try in this case)
Reboot to recovery after that.
Go to install
Choose the LOS15.1 , click on "Add more Zips" and Choose the OpenGApps ARM-64, 8.1, pico package
when the installing is done click on Sign Boot wait 2 seconds and reboot
Go thru the Setup Wizzard and it all should work.
If this works you can reboot to recovery and flash magisk, i tested with 16 and 16.7 both work here. (Dont forget to click sign boot after that)