Hi AnotherElk,
Many thanks for your reply ;
I would have expected the slot to change since I applied the update OTA, it always works like a charm…
My phone was not detected with fastboot command, and adb device listed my device with unauthorized status …
But as the adb sideload command worked, I finally resolved to applying the same update manually via adb, following that procedure , but skipping the Factory Reset step.
Also installed the GApps.
And rebooted nicely back into LineageOS, as if nothing had happened 
So, all good for now I now I know what to do if that happens again !