I feel guilty putting something so minor considering all the big issues going on, but I said the same thing with 2 and ended up never reporting it.
In version 2 you couldn’t edit length and position of appoggiaturas (or any grace note really), now you can! Yoo hoo! But editing they’re velocity seems to have no effect, especially for the ones in between.
That’s it really…
Actually you could edit grace note playback in version 2 using the properties panel, but explaining exactly how to get the results you wanted is probably not worth the effort now. Basically, if you switched on the “start and end playback offset” properties the playback became into the written length of the grace note, and you could tweak it from there.
Ah interesting, but too late now… Anyway, who wants to work like that, right?
Bollen, I think that certain Expression maps ignore velocity and use CC (like CC1) to handle intensity. This happened to me with strings. Which instruments are you writing for?
Or does this happen only for appoggiaturas?
Yes, only for grace notes and obviously on instruments that use velocity… 
Thanks for reporting this. I can confirm the problem. It’s logged and we’ll try to fix it as soon as we can.