I can't fathom the logic between engine on / off and the battery bars.
You would expect the bars to drop to 3, then the engine fires up, then when they get to say 7 the engine cuts out and you're on EV.
I use the instant MPG bar as a good indicator as the engine on / off. The engine seems to power up randomly, even when I'm showing 3 to 7 bars.
It all works OK so I'm not at all bothered.