Similar problem happened with TBC Timewalking vendor: https://us.forums.blizzard.com/en/wow/t/tbc-timewalking-vendor-doesnt-sell-items-to-level-30-characters
Timewalking dungeon queues used to be only available to level 30+, but was opened to level 10+ this patch. However the non-classic timewalking vendors were not updated correctly. The vendors need to be fixed or we will likely have this problem again for Cata TW in a few weeks.