Do effects that activate on a critical hit still activate when the hit is against an opponent wearing adamantine armor?

Question

I know from the question "Does a natural 20 on the attack roll still automatically hit if the target is wearing adamantine armor?" that a critical hit against adamantine armor will still hit, but I was curious whether it no longer being a critical hit affects features or feats that are dependent on a critical hit.

An example: Recently, in a kind of PvP session, I was wearing adamantine armor (plate), and my friend had the Great Weapon Master feat. The first benefit of the Great Weapon Master feat is listed as (PHB, p. 167):

On your turn, when you score a critical hit with a melee weapon or reduce a creature to 0 hit points with one, you can make one melee weapon attack as a bonus action.

Would a critical hit that became a normal hit still activate the bonus action melee weapon attack?

The wording of the Great Weapon Master feat is tripping me up in this case. If a player "scores" a critical hit, is that something that should affect them even if the hit ends up as a normal hit?

For the record, my view on it is that the effect wouldn't work because it ends up not being a critical hit. However, both the other player and the DM in the example I give ruled otherwise, so I'm just seeking confirmation to show them in case the situation comes up again, not trying to argue against what I believe is an obvious ruling.

Answer 1

No

If it's not a critical hit, then it's not a critical hit. The adamantine armor specifically states:

While you're wearing it, any critical hit against you becomes a normal hit.

A hit remains functionally different than a critical hit. If the mechanic turns a critical hit into a normal hit, then anything that keys off it being a critical hit won't happen as that gate never gets passed.

Answer 2

It depends whose turn it is, if you are using Xanathar's Guide to Everything

Adamantine armor provides an ability that triggers off of critical hits. Rather than preventing them from occurring, when one happens it is converted into a normal hit. Similarly, when the hit happens the attacker might have abilities like your example of Great Weapon Master which also trigger.

In regular play, there is no set way of resolving this. By default, simultaneous things happen simultaneously and when they are contradictory like this there is no resolution order that is objectively more supported by the RAW for the general case. The DM, then, has to make up something extra-legal to happen.

Xanathar's Guide to Everything, however, provides an optional rule that deals with this and most other such cases:

If two or more things happen at the same time on a character or monster's turn, the person at the game table-- whether player or DM-- who controls that creature decides the order in which those things happen. For example, if two effects occur at the end of a player character's turn, the player decides which of the two effects happens first.

If you are playing with such a rule, GWM should always trigger, since it can only be used on your turn and you, the player, decide then what order things happen in.

If you had made an opportunity attack that critically hit and had some other feature that triggered at such times, it would depend whether that attack was in response to opportunity-provoking forced movement on an ally's turn (in which case it probably triggers) or regular movement on the creature's turn, in which case it probably doesn't.