So, same version of everything.

SpongeVanilla - 1.20.6-11.0.0-RC1618-universal
BlueMap - 3.21 for 1.20.6

I get the following error on player login\logoff events, and this is probably why player markers don't show up on the map. This is not such a big deal at this time; and, it might be related to your comment regarding the API version, so, just giving you a heads up.

[10:44:33] [Server thread/ERROR] [sponge]: Could not pass ServerSideConnectionEvent$Join$Impl to bluemap java.lang.NoSuchFieldError: Class org.spongepowered.api.entity.living.player.gamemode.GameModes does not have member field 'org.spongepowered.api.registry.DefaultedRegistryReference NOT_SET' at de.bluecolored.bluemap.sponge.SpongePlayer.<clinit>(SpongePlayer.java:51) ~[SpongePlayer.class:?] at de.bluecolored.bluemap.sponge.SpongePlugin.onPlayerJoin(SpongePlugin.java:193) ~[SpongePlugin.class:?] at org.spongepowered.common.event.manager.RegisteredListener.handle(RegisteredListener.java:87) ~[RegisteredListener.class:1.20.6-11.0.0-RC1618] at org.spongepowered.common.event.manager.SpongeEventManager.post(SpongeEventManager.java:407) ~[SpongeEventManager.class:1.20.6-11.0.0-RC1618] at org.spongepowered.common.event.manager.SpongeEventManager.post(SpongeEventManager.java:437) ~[SpongeEventManager.class:1.20.6-11.0.0-RC1618] at org.spongepowered.common.SpongeCommon.post(SpongeCommon.java:123) ~[SpongeCommon.class:1.20.6-11.0.0-RC1618] at net.minecraft.server.players.PlayerList.handler$spongevanill$zie000$impl$onInitPlayer_join(PlayerList.java:1917) ~[?:?] at net.minecraft.server.players.PlayerList.placeNewPlayer(PlayerList.java:290) ~[?:?] at net.minecraft.server.network.ServerConfigurationPacketListenerImpl.handleConfigurationFinished(ServerConfigurationPacketListenerImpl.java:143) ~[?:?] at net.minecraft.server.network.ServerConfigurationPacketListenerImpl.md01a5ce$spongevanill$lambda$impl$onHandleConfigurationFinished$1$1(ServerConfigurationPacketListenerImpl.java:1118) ~[?:?] at java.base/java.util.concurrent.CompletableFuture.uniHandle(CompletableFuture.java:934) ~[?:?] at java.base/java.util.concurrent.CompletableFuture$UniHandle.tryFire(CompletableFuture.java:911) ~[?:?] at java.base/java.util.concurrent.CompletableFuture$Completion.run(CompletableFuture.java:482) ~[?:?] at net.minecraft.server.TickTask.run(TickTask.java:18) ~[?:?] at net.minecraft.util.thread.BlockableEventLoop.doRunTask(BlockableEventLoop.java:162) ~[?:?] at net.minecraft.util.thread.ReentrantBlockableEventLoop.doRunTask(ReentrantBlockableEventLoop.java:23) ~[?:?] at net.minecraft.server.MinecraftServer.redirect$spongevanill$zco000$tracker$wrapAndPerformContextSwitch(MinecraftServer.java:4693) ~[?:?] at net.minecraft.server.MinecraftServer.doRunTask(MinecraftServer.java:841) ~[?:?] at net.minecraft.server.MinecraftServer.doRunTask(MinecraftServer.java:168) ~[?:?] at net.minecraft.util.thread.BlockableEventLoop.pollTask(BlockableEventLoop.java:136) ~[?:?] at net.minecraft.server.MinecraftServer.pollTaskInternal(MinecraftServer.java:823) ~[?:?] at net.minecraft.server.MinecraftServer.pollTask(MinecraftServer.java:817) ~[?:?] at net.minecraft.util.thread.BlockableEventLoop.managedBlock(BlockableEventLoop.java:145) ~[?:?] at net.minecraft.server.MinecraftServer.waitUntilNextTick(MinecraftServer.java:792) ~[?:?] at net.minecraft.server.MinecraftServer.runServer(MinecraftServer.java:696) ~[?:?] at net.minecraft.server.MinecraftServer.lambda$spin$2(MinecraftServer.java:275) ~[?:?] at java.base/java.lang.Thread.run(Thread.java:1570) [?:?]

That is now an error caused by you using Sponge-API 11 while BlueMap-3.21-sponge is made for Sponge-API 8.2
Not a bug, just not compatible with your version.

I will probably update to API 10 next bluemap version since that seems to be the latest stable API release by sponge, that might help with compatibility in your case and fix your error.