Also in case you haven't caught the reply I made in your other thread, you really should avoid using EV mode (Auto with EV charge available or EV Now) on the highway as it really stresses the battery. If you have charge available, switch to EV Later. If the plug-in charge is depleted, EV Auto (it won't let you switch out of it in this case) will operate in the 'safer' hybrid operation.
Other than that, the other comments here are correct. The battery still has some level of charge internally like 10-15% or thereabouts for that fallback hybrid operation that it constantly cycles up and down as you drive.