Maps will be kept in memory even after disconnecting from a server.
Proof: https://youtu.be/2vcMEGn4IgI ( 1Min 32 Sec )
The Video shows multiple reconnects to a Server resulting in a OutOfMemoryError with Vanilla 1.8.6
The only possible workaround is to restart the Minecraft-Client.
This issue is not fixed in 1.9.2.
MC-46345 only refers to the maps no longer updating.
This report refers to the memory never being cleared and ultimately causing a OutOfMemoryError.