Minecraft 1.15.2
Forge 31.2.0
YAMDA 5.0.3

Description (memory leak):
Using the mining dim portal block increases RAM used on both the client and server. It will both exceed the allocated stack as within the allocated heap.

This can be confirmed by taking the following steps:
Right click the portal block to travel to the mining dimension. Right click the portal block to travel back.
Repeat this 20 times and you'll diffidently be able to see it both in game and your task manager.
Conclusion: Some of the loaded memory doesn't get unloaded.

The following is unique to Dedicated Server connections:
Description (ghost blocks):
When you travel from the mining dimension to the overworld while holding an opague block (such as dirt, nether rack, stone, etc), rightclicking the portal block will cause the player to be teleported back to the overworld and above their head a ghost block to appear. (this ghost block only appears on the client who travels to the overworld)

On the server, the following log entries can be seen:
[Netty Server IO #2/ERROR] [net.minecraft.command.arguments.ArgumentTypes/]: Could not serialize net.minecraftforge.server.command.ModIdArgument@3ba2d59d (class net.minecraftforge.server.command.ModIdArgument) - will not be sent to client!
[Netty Server IO #2/ERROR] [net.minecraft.command.arguments.ArgumentTypes/]: Could not serialize net.minecraftforge.server.command.EnumArgument@65969e60 (class net.minecraftforge.server.command.EnumArgument) - will not be sent to client!

Note that unloading the chunk and then reloading it causes the correct block to be shown on the client. (air usually)
(to confirm this quickly, try travelling to the nether and back)

The client is capable of interacting with the ghost block. The client thinks it is a portal block. Placing a block on it will replace the block with the block held by the client.

Steps to take to see this bug:
Hold any opague block, then click the portal block with it. (note that due to latency between client and server, it may appear momentarily as if the block is being placed against or on top of the block when there is a distance between the portal block and the playere, but this distance isn't nessecary). The bug can occur even by simply standing on top of the block.
Note: It maybe nessecary to have a difference in Y level between the overworld and mining dimension's arrival coordinates.

Note: I think this ghost block thing maybe related to the memory leak.