When a custom overlay is present in resource pack and is referenced by the data pack, that texture is loaded into the RAM. However, it seems like the game never clears them out even quitting the world, nor does it appears in the memory counter in the F3 debug screen.

How to reproduce

1. Open up Task Manager to monitor the RAM used by Minecraft.
2. Prepare a lot of custom overlay textures, and display them one by one in game on screen.
3. Note that the RAM used by Minecraft, according to the Task Manage, increases rapidly. However the value is only a little according to the debug screen.

The attached recording.mp4 has this process recorded. In the video, I have a 1080p frame played on every game tick (20fps).

4. Now quit and save the world. Check the Task manager, and the RAM is still not cleared. （See Quitting the World.mp4 for this one）

Expected Result:

The game should clear the memory used by custom overlays out of the memory.

Additional Information:

1. Exit the game will free the memory
2. Unload the resource pack will free the memory

Example

An example resource pack and data pack is provided here: https://www.mediafire.com/folder/o8xf6r876p12n/Bug

This is the same pack in the video. To use it, install the resource pack and data pack. Execute 

function do:get

to obtain a special item, then equip this item on your head.

To stop, simply take it out of the player's head slot. Made for 1.21.3.