Mine has always been the same way. It starts to become slightly neutrally buoyant if it has been used in the last day or two and absorbs a little water.
You can see parts of mine being above the water:
The positive buoyancy will pull the ROV backwards if it gets deep enough. It gets hard to stay in position since a constant horizontal thruster input away from the spool is required, but it’s always obvious which direction the home position is. 
I have always assumed getting dragged backwards was a normal part of “Tether Drag”
like 34:47 here when I took my hands off the controls to adjust the Ping360 settings
and around 17:40 here:
