The arpeggiator in Serum 2 is bypassing the master on/off for the Serum 2 channel and sending sound to the master. When i mute the Serum 2 instance on the channel rack the clip/arpeggiator is still playing through to the master, Not sure if this is something on image lines end or xfer so ill comment on both.
Update: FL got back to me, i suppose this is the way in functions.
"The mute LED in the Channel Rack in front of the plugin does not mute the plugin itself, it mutes the notes sent from the Piano Roll to the plugin, so you are still able to play it with a MIDI keyboard or similar.
Notes from within the plugin can't be muted, as these are not notes sent to the plugin via FL Studio, so these will still be able to play fine."