Step 1: Create a marker in the overworld
[13:36:58] [Server thread/INFO]: Created new marker #1 “Overworld”@(-768, 304)
[13:36:58] [Server thread/INFO]: Sent markers data #764 to player Watherfoxx

Step 2: Create a marker in another dimension
[13:37:22] [Server thread/INFO]: Created new marker #1 “OtherDim”@(-4749, -5403)
[13:37:22] [Server thread/INFO]: Sent markers data #764 to player Watherfoxx
The marker created in the other dimension is not visible, whereas the one in the overworld is.

Step 3: Disconnect and reconnect
The first marker placed (overworld) has disappeared and the one in the other dimension is now visible. Since they had the same ID, the 2nd has replaced the first.

ID incrementation is specific to each dimension, which creates problems.
I'd like to take this opportunity to mention that the same applies to Atlas ID incrementation.

Minecraft version: 1.16.5
Antique Atlas version: 6.2.4
Forge version : 36.2.41