Do players still earn xp when all masteries are maxed but there is also a locked mastery?

Ok, so I thought the answer was no, it's being wasted because in game it says 0/0.. but then I found this dev response on reddit that says it's being saved in a hidden xp bar for a player who is soft maxed due to a lack of mps to complete full mastery.

So this should also apply in this situation with the release of skyscale for people who had max masteries before, yes? If a developer could confirm this interaction is correct.. then one of my biggest concerns about engaging in the new content before unlocking skyscale is completely gone, this would help immensely.

https://www.reddit.com/r/Guildwars2/comments/ac6qcy/new_pf_player_cant_reach_any_mastery_points/ed67krf

"My exp bar is maxed, so I feel like I'm wasting playtime too."

"You don't have to worry about this. XP you get when you have all available mastery XP tracks maxed goes to a hidden XP bar which will give you spirit shards once you've unlocked all the masteries. This is not well explained, and I actually just found out about it myself yesterday"

In my case i can't exp since the release. The skyscale mastery turn my exp bar into an empty one and it looks like i have to get the mount and all masteries to exp again in PoF/S4.

I think that Dev response has been misinterpreted, or the Dev didn't explain it well, or was himself confused about how it works. No one has ever commented on gaining a bunch of Mastery XP when a new Episode released (with the associated new Mastery Track) before.I suppose we will find out for sure when those that have maxed out their PoF Masteries unlock the Skyscale, and thus, its Mastery Track.

That dev quote has been known for a long time already to be inaccurate. Most probably because the dev wasn't speaking from personal knowledge, but retelling (inaccurately) what he has heard from someone else.

So, the answer is exactly as you thought - you earn no xp in such a case, and it's not "banked" in any way, but simply lost.