Possibly a bug: sorry if it has already been reported.
open the project
grace note 8th line.7z (397 KB)
and delete any grace note
You’ll see: the 8th line vanishes, notes stay in the same position, playback is the same.
I’ve also noticed that you cannot tie grace notes together, but maybe this is by design - and not a real issue for me - I can notate things differently.
You can’t have dotted grace notes either. There are a few of those still left over from the 18th century!
The 8va line needs to be anchored to a real note. The issue is if you delete a grace note it doesn’t leave a rest behind and so there is no anchor and the line disappears. Yes it still has an effect so it really is still there, but with no anchor it doesn’t know how to display itself (I am making assumptions since I don’t know how the code actually works. But I’ve also run into this before.)
If you really need to 8va line to cover the grace notes, the only way to do this is add a bar before the first and anchor the line on a note in the prior bar, and then delete that note. The 8va line will then anchor to that rest. Alternatively, anchor the 8va line to the first real note and leave the grace notes outside of it.
Or just don’t delete any grace notes after you’ve added the 8va line.
thanks for your explanation, that’s how it is apparently. Still I think this behaviour should be changed to something like: move the anchor to the nearest note (priority 1. the grace note before (if exists), 2. the grace note after (if exists) 3. the normal note the grace note was attached to, and things get mixed up if you consider grace notes in another voice 
), or delete the 8va line altogether. In my case I didn’t notice at once the 8va line was missing and later it was a little bit annoying to fix it.