The master level fletcher trades can now sell tipped arrows with the thick, mundane, and awkward potions as they are included in the tradeable potion tag. This did not occur in 1.21.11 as potions without effects were excluded from the available tipped arrow sell trades.
This information is specified in data/minecraft/tags/potion/tradeable.json.
I could replicate this bug using the following command:
A Fletcher selling the effects defined under the data/minecraft/tags/potion/tradeable.json was sold.