I've had this bug with both flash and HTML5 send fleet to attack a target the target vanishes then cannot join the attack.
Targets are on a timer, and after so long they are reaped from the map. This is basically a scenario where your fleet engages with the target as it is reaped from the map and the game can't figure out what is going on. I would recommend sending a ticket in when this happens.
I have been using hardware acceleration with all of these problems and some others which are now coming to light. The range rings are not showing the colour indicator for battle does not show the colours of damage show in the bottom right menu are messed up the gold on gold when running with a crew the list goes on. The timer problem is still there no matter what I do.
graphics bug surfaced and its happening all the time when returning to the game to check on repairs
This is by no means new and has been going on for a long time. For some reason the health bars decide to move faster than
they should. It is most noticeable when you leave the game open in the background for a while. Stopping and restarting the repair syncs the health bars back up
correctly, refreshing can clear it up as well.
Targets are on a timer, and after so long they are reaped from the map. This is basically a scenario where your fleet engages with the target as it is reaped from the map and the game can't figure out what is going on. I would recommend sending a ticket in when this happens.
Best I can suggest here is refresh and then try again. Though at this point I would hope that timer has long since completed.
Enabling hardware acceleration in your browser settings should correct most of this.
I think the hardware acceleration that I mentioned on the last one will help with the sprites remaining once destroyed as well.