It's probably electrical. Weren't these transmissions recalled because they would go into park while driving if there was an electrical hiccup so they were reprogrammed to go into neutral instead(as if that's a great alternative)? If I were a tech that is where I would start, and hence why they started at the battery terminals.
Found it:
https://www.autoblog.com/2019/04/25/honda-odyssey-minivan-recall-transmission/
Sound like a new battery may solve it, but I'd let the dealership figure that out on their own.
Are you having difficulties getting the dealer to inspect and fix? I'm usually game for confrontation, maybe you have your own approach.
Storm into the dealership at a busy time (not near closing time) having dressed very nicely and parked the sparkling clean van right in front of the sales doors where everyone can see it each time you loudly point to it and raise hell in the sales area. Start with YOUR salesman, not the service department which is conveniently separate from the sales floor, and refuse anything short of them taking the van and giving you a loaner until they solve the issue, or the even more challenging lease take-back. You have to keep your composure though or you just come across as a crazy irrational person.
Salesman: why don't you go over to service about this?
You: (
starting softly and getting progressively louder) We're going to take care of this now, and you sold me this thing (pointing at van),so you're in it with me. Why don't we have service come over here and the 3 of us will work this out right here (keep him on the floor with you, don't let him run away, so you motion to another person to have THEM get a service person over to the two of you). I'm a busy person, but I cleared my afternoon for just this, so I'm not leaving until we work this out. That van (pointing at van) is such a hot mess Honda themselves flew a tech out here to look at it and even they couldn't make it work, but we're going to solve this right here and now and begin the "airing of grievances" listing everything wrong about the van.... Eventually a service person will show and you get to do an encore presentation on the sales floor.