I personally didn't experience this bug, but 2 of the 10 high dragons went invisible before dying, had to kill them by spamming melee attacks where they were last seen 
Btw my party was Cass, Vivienne, Varric, tempest archer. In the late game I gave them all a gear piece with +5 guard on hit, used the appropriate resist potion and all the dragon fights were a breeze. I was controlling Vivienne, the rest was set to just autoattacks and buffs. If it was a dragon with weakness against fire, it died ridiculously fast (15k+ crits with Fire Mine).
But if someone wants to make the fights easier, use tempest's Thousand Cuts with assassin's Mark of Doom. I was doing some testing on nightmare on the last dragon (lvl 23), I myself was lvl 24. When I buffed my archer with everything I could think of, taunted the dragon with Cass, applied Mark of Doom, the dragon died just from the Thousand Cuts. That is 315k HP instagibbed. When I forgot one buff I had the time to detonate Mark of Doom... for 300k dmg.