I have tried various commands with Alexa and pretty much reproduce what you have found, namely that one can raise and lower the shades by asking Alexa to turn them ON or OFF. However, nothing I have tried works to STOP the shades. I tried “turn Shade on 50%” and Alexa rather peevishly said that she does not recognize that command. Also nothing happens with a STOP command. This is not surprising given that there is nothing in the Aeon Motor Controller software or the Rollertrol motor firmware that would indicate shade position. In the Raspberry Pi system Rollertrol has developed, shade position is determined by first measuring the time for the shade to go from full UP to full DOWN, then the system runs the motors for half that time for a command of 50%, and proportionally for other positions. Of course, there is nothing like that for the Smartthings hub until someone writes the code.
So I use the ST app to control the shades and to stop them midway if I am away from home. If I want, I can check the positions visually using the web cam I have in the room, but will experiment with a crude version of the Rollertrol system mentioned above to stop them at an intermediate position. It takes about 16 sec from full UP to full DOWN so counting (I said it was crude.) 8 sec should roughly give a mid-window position. Anyway that’s my plan until someone comes up with a better idea. Or until I get the Raspberry system.