Disappearing HP bars

I keep meaning to post this, but it occurs so frequently I tend to forget about it until it annoys me again. HP bars disappear and don't show the damage they've taken during battles and in replays.

 

What are the circumstances of the disappearance? Are they showing and then disappearing over time? Do they all disappear at once?

 

Happens to me sometimes when attacking. Some buildings and defenses have missing HP bars while being attacked.

 

Only some of them. Not sure on circumstances around it yet.

 

Gather more information regarding the circumstances and then tag Sir Bugsalot

 

For me, the buildings with those cannot be destroyed.