Tank freezing in dungeons

I play as a Conjurer. I know my role. I heal heal and even more heal.

However, for the past two days, whenever I enter a dungeon using Duty Finder, I can no longer play my role.

At some point in each dungeon run, the tank is displayed on my screen as just "standing still" while the rest of the group moves on. However, I know that's not the case. Agro lines immediately draw all the way across the map to where the tank is displayed as standing still for me.

The problem is that the tank becomes un-targetable for my heals unless I stay WAY BACK where the character appears to be standing. Meaning I can't help the rest of the group.

I imagine I can't be the only one suffering from this issue. But unless my search-fu is really weak...I can't seem to find topics addressing it.