I'll link to the Issue reported on Curios Issue Tracker as we are not sure which mod is the main cause for this to happen. But a combination of Curios, Inventory Essentials and Inventory Sorter.

TheIllusiveC4/Curios#391

PS: The problem doesn't happen until both Inventory Sorter and Inventory Essentials are present will post in both.

Does anyone know what the Hexerei coffer string is? I added "hexerei:coffer" to the containerBlacklist and it still sorts to main inventory.

Does anyone know what the Hexerei coffer string is? I added "hexerei:coffer" to the containerBlacklist and it still sorts to the main inventory.
I don't know the mod Hexerei, but I can suggest 2 ways:

1. Try to use /give command to check what string gives you the same item that you are searching for
2. Looking the mod code, I have found:

1. hexerei:block/coffer_container
2. hexerei:block/coffer_chest
3. hexerei:block/coffer_block
4. hexerei:block/coffer

Maybe one of these is what you are searching for

Does anyone know what the Hexerei coffer string is? I added "hexerei:coffer" to the containerBlacklist and it still sorts to main inventory.

Try F3 + H

F3 + H - Advanced Item Tooltips (Basically shows Item IDs)

Then when you hover over your inventory items, you will get some extra info about them.

You can blacklist container in config.
edit string list "inventorysorter:dummyplayercontainer"

I'll link to the Issue reported on Curios Issue Tracker as we are not sure which mod is the main cause for this to happen. [Vou criar um link para o problema relatado no Curios Issue Tracker, pois não temos certeza de qual mod é a principal causa para isso acontecer.] But a combination of Curios, Inventory Essentials and Inventory Sorter. [Mas uma combinação de Curiosidades, Fundamentos de Inventário e Classificador de Inventário.]

TheIllusiveC4/Curios#391 [TheIlusiveC4/Curios#391]

PS: The problem doesn't happen until both Inventory Sorter and Inventory Essentials are present will post in both. [PS: O problema não acontece até que o Inventory Sorter e o Inventory Essentials estejam presentes e serão postados em ambos.]

I only have the Curious and Inventory Sorter mods, and that's enough to cause the problem

This is confusing as by default, the curious mod doesn't actually add any more slots, or at least that's what's I found in my testing

Could you provide any more information for me to try and reproduce this in my environment

I could help you to reproduce the issue.
With these relics (https://www.curseforge.com/minecraft/mc-mods/relics-mod relics-1.21.1-0.10.7.5.jar)
and this album (collectorsalbum-neoforge-1.21.1-2.2.1.jar)
and this crafting stick (crafting_on_a_stick-1.21.0.1.jar)
and time in a bottle (tiabcurio-neoforge-1.21.1-3.0.1.jar)
and this backpack (diamond version https://www.curseforge.com/minecraft/mc-mods/sophisticated-backpacks sophisticatedbackpacks-1.21.1-3.24.15.1250.jar)

sorting this part of curio inventory, relics are overwritten and lost forever.

After sort:

This helps a bunch! Thanks for outlining how you're causing this to happen. I think I know why it's happening and the solution is likely pretty simple.

I just need to find some time to fix this now, should be relatively soon. Keeping this in my unread notifications but I have seen it!

You can blacklist container in config. edit string list "inventorysorter:dummyplayercontainer"

I tried doing this but it didn't work. the sorting is still able to apply to the curios slots and delete everything

I added this instead to both the container and the slot blacklist string

"curios:curios_container"

It works but I don't know which string is the appropriate one to block. for now this makes the sorting ability disabled when the curios slot is opened. it looks like it treats the curios slots menu as a container that is opened instead of a slot for player equipment.

Whilst I'm working on this, here are some updates,

The container blacklists aren't identical, specifically, curios requires you to blacklist the their slots which would be top.theillusivec4.curios.common.inventory.CurioSlot which I've just tested and can confirm this works to stop voiding items

This also seems to happen with other mod containers. Hexerei coffers do it one better because sorting the coffer moves the contents to the main inventory, overwriting what is already there.