You are confusing two things. If you set a flow amount in your slicer, it will output each extrusion gcode command multiplied by that value. If you change the flow rate in OctoPrint, it does not change the gcode commands, but instead it sends an M221 command, which instructs the firmware to multiply all extrusion values set in the gcode on the fly.
If you set your flow in your slicer to 90% (or 0.9) and then issue the M221 command from OctoPrint, the effect would indeed be the same as setting it to 0.81 in your slicer. However, if you send the M221 command again with the 90% multiplier, the result will still be 0.81, NOT 0.72.
This has not changed between
1.4 and 1.4.1 1.4.1 and 1.4.2. What has changed is that the percentage is now displayed write-only.