We have an issue that has shown up a few times, but doesn't seem incredibly common, so I wanted to share our experience.
My wife was driving our brand new (purchased 4/19, now has about 1200 miles) 2023 LE to pick me up about 90 miles from our house on Saturday evening. The end of the drive goes from 4500 to 8500 feet. About half way up the climb, the message "hybrid system malfunction" comes on the dash and the car slows to 40 mph. After a couple minutes it slows down to 20 and my wife is now frightened. She (very fortunately) found a place to pull over. A friend drove me down to her and we duly noted that the owner's manual says to stop driving and have the car taken to the dealer. Fortunately (again) we have AAA and were able to have it towed back to our house (too late to take it to the dealer). Sunday (the sales department is open) we have Toyota Roadside Assistance (really AAA again) tow it to the local dealer (not where we bought it, as they charge $3000 extra).
This morning the service advisor calls and says they will take a look at it. This afternoon he calls again and says the warning wasn't on any more. They pulled the code that had the warning and noted it, but they were ready to give us the van back after nothing happened on a test drive. I told him there was no way we were taking the van back without them doing anything to it, given that it had stranded us. Perhaps he didn't realize earlier that we had been stranded by our brand new Sienna, but the idea of just giving a car back without doing anything after such a serious warning is ridiculous.
Anyway, this is the first post and I will update it as we get more information, but this is a bit souring given that our reason for going with Toyota is they are supposed to be incredibly reliable and they have been making hybrids for over 20 years.