EDIT (3 July): We have now rolled out a new update 30.0.1 for all players, which should fix this issue. Please update your game!

30th of June, 07:00 AM UTC: There is a known issue with Troops during gameplay Battles in the Sand Empire Summer Event.  We are currently investigating the issue here. We will update this announcement with any new information
.
You may also follow the thread on our Community Forum regarding this issue, and to see any updates posted there: Sand Empire Bug

We hope to fix this issue as soon as possible. Our apologies for any inconvenience caused by this issue!