I don’t meet the requirements for that quest somehow, but I remember that it should properly reset each time next Timewalking comes around, so it’s not a problem with me completing it before.
The WOD Timewalking vendor is also missing, for Alliance at least, which might be the root cause.
My level 50 mage has the WoD experience buff showing but the option to queue for TW dungeons is not appearing (not even greyed out). The WoD TW vendor is also not showing for me either. I haven’t purchased Shadowlands.