I played the event raid of this week.
There were more than 1300 gems but it became 139 gems after the event raid. 1200 gems disappeared.
I thought I bought something by mistake but all imps were still working, training was ongoing and my resources were full.
It is impossible to buy boost by mistake for 1200gems.(many clicks for 1200gems)
I am very sure I had more than 1300 because I kept it for library upgrade.
I restarted app and iPad, also re-logged in but the gem was lost:(
Already reported to Chris.