Without removing/purging but simply not using entrance through "dpkg-reconfigure entrance" and enabling lightdm solves the issue i.e lightdm turning off plymouth is not overridden.
The trouble is that this plymouth issue does not happen all the time. Looks like it only happens when entrance has a new/changed configuration.
Which would also be the case on a first boot if it was already there after an upgrade. 
So: For now it's not advisable to keep "entrance" installed when upgrading i.e use it on first boot until the issue is fixed.
Topic
It seems to be going off topic but is actually a side step. I ran into this whilst looking into the "unable to reboot/shutdown" issue i.e setting Entrance to auto-login and back again.
So: IMO that shutdown/reboot issue is related to "autologin" being used in the live/install/upgrade session.
