Getting the messages below at login. May be only a fraction.
Probably a lot of "Hidden Quest Triggers", not sure if it's useful reporting these, since it's not clear when these were triggered the first time. There are a few genuine quests in there :-)
Get ready for it...

Let me know if I can be of any help with this.

Not everything from here was added, @Darkal31 will go over this list later or I will change it to the task list instead of screenshots (maybe).

Seems I'm not getting these messages anymore, even with the options to report them checked.
Can I reactivate these somehow, so I can keep an eye on the progress and add some myself? :-)

Hm, I think it might be Blizzard's change, they started sending these events not only on login but also on battleground load but maybe now they scaled down stuff so you can't see it? Honestly I haven't paid much attention to login spam lately, but maybe it was something @ImUnicke changed? 🤔

Yea I disabled the on-login Quest reporting spam because it's not helpful to know what the quest ID's actually are.
If you really want all quests you have completed on a character to print into chat on login/reload, just remove the app.IsReady and from this snippet in AllTheThings.lua

In future news, I want to add a new window which can be opened which simply lists every QuestID in numeric sequence for easy visibility of all possible Quest ID values and whether they are completed/sourced/etc. so that functionality (once existing) might make it pointless to print all the completed quests in chat on login. But that doesn't exist yet...

Adding more to the list from discord errors

[18:16] Quest completed #35661 (Not in ATT vSL-2.3.4a)
[18:16] Quest completed #35976 (Not in ATT vSL-2.3.4a)
[18:16] Quest completed #45393 (Not in ATT vSL-2.3.4a)
[18:16] Quest completed #50460 (Not in ATT vSL-2.3.4a)
[18:16] Quest completed #52296 (Not in ATT vSL-2.3.4a)
[18:16] Quest completed #54325 (Not in ATT vSL-2.3.4a)
[18:16] Quest completed #54401 (Not in ATT vSL-2.3.4a)
[18:16] Quest completed #54716 (Not in ATT vSL-2.3.4a)
[18:16] Quest completed #54773 (Not in ATT vSL-2.3.4a)
[18:16] Quest completed #54953 (Not in ATT vSL-2.3.4a)
[18:16] Quest completed #55480 (Not in ATT vSL-2.3.4a)
[18:16] Quest completed #56607 (Not in ATT vSL-2.3.4a)
[18:16] Quest completed #56727 (Not in ATT vSL-2.3.4a)
[18:16] Quest completed #56944 (Not in ATT vSL-2.3.4a)
[18:16] Quest completed #56945 (Not in ATT vSL-2.3.4a)
[18:16] Quest completed #56949 (Not in ATT vSL-2.3.4a)
[18:16] Quest completed #58576 (Not in ATT vSL-2.3.4a)
[18:16] Quest completed #59133 (Not in ATT vSL-2.3.4a)
[18:16] Quest completed #62396 (Not in ATT vSL-2.3.4a)
[18:16] Quest completed #62397 (Not in ATT vSL-2.3.4a)
[18:16] Quest completed #62979 (Not in ATT vSL-2.3.4a)
[18:16] Quest completed #63426 (Not in ATT vSL-2.3.4a)

I can only assume this has been fixed or is in a workable state by now. If not, please refer to the (retail) errors channel on our Discord to place a more detailed report. :)

Oh, yeah, there are many more I believe! So many that they simply get clipped lol. Unfortunately there is no easy way to identify what are these, you'd need to manually check every id using sites like wowhead or icy-veins. @Pr3vention was doing this work while questing through expansions. He was using an addon to extend chat history to 5k lines, /reload and then checked ids from the top
As you said many of them are hidden triggers so it's even harder to find any info online.

Think once @Darkal31 is satisfied with the on-going quest history additions, this issue can be closed.