Datapacks that use custom chest inventory broken on Paper

[Nystrata]

This involves the Waystones datapack or any other datapack that uses chests for custom chests inventories.

Broken on paper-26.1.2-18 and stable paper-26.1.2-53
Works on older paper-26.1.2-17 and vanilla.

Expected behavior

Interact with a Waystone (from https://modrinth.com/datapack/waystones-data-pack/version/V.3.5.1)
The waystone will list other waystone destinations in an inventory menu

Observed/Actual behavior

Interact with a Waystone
The waystone fails to list other waystones

Steps/models to reproduce

Install https://modrinth.com/datapack/waystones-data-pack/version/V.3.5.1 into the world's datapack folder
Run paper-26.1.2-18 or later (including the latest paper-26.1.2-51)
Place a waystone (according to "Place a waystone" of https://modrinth.com/datapack/waystones-data-pack)
Place another waystone
Interact with either waystone

Plugin and Datapack List

plugins
[03:28:52 INFO]: ℹ Server Plugins (11):
[03:28:52 INFO]: Bukkit Plugins:
[03:28:52 INFO]: - CoreProtect, DiscordSRV, Essentials, Herobrine, LuckPerms, packetevents, PVPToggle, TreeFeller, ViaBackwards, ViaRewind
[03:28:52 INFO]: ViaVersion

datapack list
[03:29:03 INFO]: There are 13 data pack(s) enabled: [vanilla (built-in)], [file/bukkit (world)], [file/Coordinate Bar-nyscustom (world)], [paper (built-in)], [file/KawaMood's Waystones in Villages_nys_1_21_11_patch (world)], [file/bat membranes v1.0.17 (MC 26.1).zip (world)], [file/graves v4.1.0 (MC 26.1).zip (world)], [file/mini blocks v1.1.6 (MC 26.1).zip (world)], [file/pk_better_item_frames_V.3.0.4_mc_26.1.zip (world)], [file/pk_no_creeper_grief_V.3.0.2_mc_26.1.zip (world)], [file/pk_waystones_V.3.5.1_mc_26.1.zip (world)], [file/unlock all recipes v2.0.17 (MC 26.1).zip (world)], [file/ArwensFurniture2_1.2.0_Datapack.zip (world)]
[03:29:03 INFO]: There are no more data packs available

Paper version

version
[03:33:13 INFO]: Checking version, please wait...
[03:33:13 INFO]: This server is running Paper version 26.1.2-51-main@5d62ec0 (2026-04-27T16:54:06Z) (Implementing API version 26.1.2.build.51-beta)
You are running the latest version
Previous version: 26.1.2-17-9b1dd57 (MC: 26.1.2)

Other

Hello,

I've been running a Paper 26.1.2 test server that uses this datapack version:
https://modrinth.com/datapack/waystones-data-pack/version/V.3.5.1

The datapack works by having a barrel open a custom inventory menu.

I bisected between paper-26.1.2-15 and paper-26.1.2-20, and it seems like the culprit is paper-26.1.2-18:
As of paper-26.1.2-18 and paper-26.1.2-51 (current latest), this is broken.
Reverting back to paper-26.1.2-17 (or using vanilla), this works just fine

This may be related to the chest-related change for paper-26.1.2-18

Delay open/close callbacks for chests

[KawaMood]

More generally, this change affects any data packs / (also plugins?) using containers for GUI, custom extra inventory, or any other reason, not specifically mine. The issue is broader than it could sound here.

[Nystrata]

Confirming that this is still broken as of Stable 26.1.2-53

This is from a new server with no plugins and just the datapack:

> version
[16:10:21 INFO]: Checking version, please wait...
[16:10:21 INFO]: This server is running Paper version 26.1.2-53-main@39a1aa5 (2026-05-01T07:50:35Z) (Implementing API version 26.1.2.build.53-stable)
You are running the latest version
> plugins
[16:10:22 INFO]: ℹ Server Plugins (0):
> datapack list
[16:10:25 INFO]: There are 4 data pack(s) enabled: [vanilla (built-in)], [file/bukkit (world)], [paper (built-in)], [file/pk_waystones_V.3.5.1_mc_26.1.zip (world)]
[16:10:25 INFO]: There are no more data packs available