I think this issue has been reported before but it is still not solved.
When killing an enemy the card remains on the field, not only that it sometimes happens with the hometeam as well.
I can’t say when it happens because it seems to be a random event. It happened to me while playing Dungeon, Raid, Explore, PvP, Petrescue… Well in every part of the game even in the newly added Halls…
I play on Steam/PC
Had this happen again as well a couple days ago. I was playing PVP at the time. PC/Steam too.
Even better: if this happens to a Gnome, you don’t get the treasure. If this happens to a tower, you don’t get credit for killing it.
4.0 is at least the third version of the game with this bug, it’s existed since the Great Unity Patch that broke everything. At this point I think it’s just a feature: sometimes because RNG, you don’t get credit for your kill. We’ll call it a “Chaos Barrier”.
Better? So you think not getting rewards is better??? 
Hey, unfortunately this is a known issue which we’re currently investigating.
For more information (and a list of info that we’d find helpful in helping to pinpoint the issue), please see this article:
https://gemsofwar.zendesk.com/hc/en-us/articles/360000305315-Enemy-troops-do-not-die-and-the-game-is-frozen
If you’re missing rewards due to this issue, please contact support so we can help you.
So I had some issues today whilst playing the class event and I did what was suggested. I did send a message to CONTACT SUPPORT.
Result: I got a mail back saying:
We aren’t able to respond to this type of request as our team specialise in Technical Support, so we’ve closed your request.
Instead, you can report your bug here:
The development team frequently reviews posts on the forums and our QA/Support team has a process to review these requests. We may not respond to your post, however your report will still be noted. If you chose not to post your report again, we’ll still read your ticket here but we won’t respond.
Thank you for helping technical support focus on devoting time and resources to helping players with technical problems!
I found your ticket and replied 
And might I ask how you replied? Because if it’s by mail I haven’t received one.
Please go back to our Help Centre here:
https://gemsofwar.zendesk.com/hc/en-us
Make sure you’re signed in, select your name in the top right and then my activities to see your support tickets and my reply
I’ve been wondering what the hell that was! Had this happen a fair bit for months now. Last occasion as approx 5 days ago.