New to the forum, found it while searching for issues with the 2018 Accord climate control. I too am experiencing issues (2018 Touring 2.0L). Car heats up just fine, but after 30 minutes or so, starts blowing cold air out of the side window defrost vents on the top of the dash. I've measured the temp coming out of the vents with an infrared thermometer; with the cabin temp set at 72-74 (in auto mode) the air coming out of those vents is in the high 50's, whereas the other vents are mid 70's to 80's. Result is the top of the legs and feet start to feel chilly. Manually selecting the combined dash and footwell vent distribution seems to help, as the window defrost vents now blow warmer air. Took the wife on a trip recently and she commented her feet and legs were cold without me having first told her about the problem.
Took the car to the dealer, and they tested the temps against an identical model off the lot. Said the readings from both vehicles were within 10%, which is considered normal. From what I saw of the readings, which the service rep was not crazy about showing me, the temps were lower on my car compared to the one from the lot. The rep followed up with Honda's tech support line - they want him to cold soak the car overnight and then run the test again against another vehicle. This sounds like a lot of nonsense to me, as it has nothing to do with whether the car sat overnight at all; it always occurs 30-45 minutes into a drive when it's cold outside. The rep also said I could do nothing and wait for more people to complain to Honda, at which time they might take it more seriously.
Anyways, that's my story so far. I noticed the last post was November. Anyone else have any more info/progress to share on their issue? Thanks.