Even though I've picked up the water talisman the objective related remains unchecked and the mission won't end when I leave the level. Thankfully I had saved right before entering the tomb so I thought I could reload and try to pick it up again but no luck this time either.

This is originally Thief Gold that I've "downgraded" to TDP thanks to the latest "GoldtoDark" patch, and yes I did apply Tfix 1.24 as well (but no other mods). I do know that the water talisman is not in this level in Gold, perhaps this is the reason the game won't acknowledge that the damn item is in my inventory?

I can't find anything online about this specific issue...

Sounds to me like a scripting error in the GoldToDark patch. Check if the author is still active on TTLG and report the issue.

if you have used TFix and then GoldtoDark then your game is probably broken, you need to apply TFix last if you want this to work.

What you are saying doesn't make sense to me.
I have installed Thief Gold, applied the GoldtoDark patch and applied Tfix afterwards to TDP.

Isn't that how it is supposed to be done?

I've tried everything and it still won't work.
If someone could tell me how to start the next mission without having to complete Lost City that would be much appreciated, though I don't know if the next one will work, and the ones after that...

yes, that's how it is supposed to be done, so no idea what went wrong there.

Alright, I just downloaded an obsolete version of TDP off a french abandonware site, patched it to 1.33, patched it to english, applied Tfix, imported my lost city save, picked up the water talisman for the 94786th time and unsurprisingly it did not work.

So I'll have to use that trick Plokite_Wolf posted (thanks a lot by the way). Thankfully money does carry over to the next mission...

I was able to replicate and fix - patching up with TFix 1.24c or newer should resolve the issue (though you'll probably need a save from before you picked the talisman up).

Edit: nevermind.

Edit: deleted