There is a feature in Bluemap called live player marker which shows the location of online players on the map. This feature worked properly with real players but will continue showing fake players after they are logged off. I asked the developer of Bluemap and they responded that they use standard JOIN and DISCONNECT events provided by fabric api to add/remove player markers. In the version 1.20.6 I can use this feature without any problems but when I updated to 1.21 this problem appears, it also appears in 1.20.4.
I wonder if there is an issue within carpet mod that fake players don't trigger the JOIN and DISCONNECT event properly?

See the attachment for the mod list and carpet config file.

I'm having the same issue, and this is the Bluemap developer's answer.

BlueMap is using the ServerPlayConnectionEvents.JOIN and ServerPlayConnectionEvents.DISCONNECT events provided by FabricAPI to detect players joining and leaving. So right now i'd say this is more an issue with the mod which is adding the fake-players, because it seems like it is triggering the JOIN event but not the DISCONNECT event of FabricAPI.

BlueMap-Minecraft/BlueMap#598