No. Theft requires intent. If we accept that it's incompetence and no one actually intended to apply the fee to an electric vehicle then it is not theft.

If no one intended to apply the fee to an electric vehicle, then why was the fee applied to an electric vehicle? Moreover why did it happen numerous times? And why was support not immediately like "oh crap lemme fix that"?

I think that a big part of Hertz’s business model is to deliberately overcharge as many of their customers as possible.