So I have a scenario where on one of my characters I got to a new renown level, but instead of seeing the NPC that gives one of the rewards unlocked for the unlocked renown level, I bought all the rewards from the Quartermaster (the NPC) that sells all the rewards for a given faction. When I then walk over the NPC that sold the rewards, I can see via ATT that it has a quest for me but it won't give it to me.

After some testing, it looks like NPC won't offer their renown quests if you have all the rewards the quest would otherwise give you, and if you have one of the rewards, then that as an option does not exist for turning in the quest.

In my original case, the following quest became unobtainable:
https://www.wowhead.com/quest=72328/our-ferocious-armored-steeds

Because I had bought and learned both of:
https://www.wowhead.com/item=201425/yellow-war-ottuk
https://www.wowhead.com/item=201426/brown-war-ottuk

Which I bought from this NPC:
https://www.wowhead.com/npc=193006/murik

Likewise, ATT also claims that Murik offers me the following:
https://www.wowhead.com/quest=72875/dragon-isles-supplies (Renown 5)
https://www.wowhead.com/quest=72389/dragon-isles-supplies (Renown 8)
https://www.wowhead.com/quest=72392/dragon-isles-supplies (Renown 11)

Which is not available from the NPC, which I assume is related to a similar issue to the above mentioned one.

I think what ATT should do here is to mark these quests as automatically collected if their rewards has already been collected as it otherwise becomes impossible to get them on the WoW account (since most of the rewards are account-wide).

Fixed

A year later - I just slowly got some rep on my alt over time and today I reached renown 30 with the Iskaar Tuskarr and this quest did not pop from Tatto (even after a relog).

So for https://www.wowhead.com/quest=72328/our-ferocious-armored-steeds and other quests that grants mounts from renown gains, (and manuscripts) are all unobtainable and basically similar to the once per account kind of quests from BFA

Yeah, we know about this. Might be helpful to add a note on the items. I'm not sure we should mark the quests as completed because you don't complete them.
@Darkal31, thoughts?

@ImUnicke plans to add lockcriteria for learning things for these renown quests in the future.

Also dragon isles supplies quests should be doable in party-sync.

I did some further testing with alts and this limitation of the quest seems to only apply to mount quests, other quests such as pets and so on, will still be offered, but mount ones will not

It also applies to quests which reward cosmetics as well I think. For those we don't technically have the proper data handling to accommodate, so that will be additional logic as well.

I did some testing with Party Sync and as far as I could tell, probably all quests for Dragon Isle Supplies that are at the same Renown level as any "primary" unlock are not doable. Quests for Dragon Isle Supplies that do not have a primary unlock at the same Renown level will appear on party sync.

I think this is a design flaw by Blizzard for completionist reasons, also tried a bug report. I have little hope that it will be changed though.

Dragonflight unfortunately has many such unreachable quests on a single character