Exactly what it sounds like, I know this is just a visual bug, but it is a little annoying. Quark adds the addition to display the vanilla survival HUD when riding a horse, which includes the TAN thirst bar. It seems like it would be relatively easy to fix, just needs some cross mod communication with Quark. If the thirst could be below the horse health that would be nice, it just needs to move the horse health up an extra level if Quark is detected when minecraft loads up during initialization.

Here's a screenshot depicting the issue:

Note that I had originally reported this to Quark's repository, but Vaskii said that this is an issue on TAN's side. Here's what he said how to fix this visual bug:
"TAN can fix this on their end by registering RenderGameOverlayEvent.Pre for HEALTHMOUNT, and moving it up a bit when the food bar is enabled (GuiIngameForge.renderFood)"

Also he said that @Adubbz should be contacted for this issue, but it seems that @Glitchfiend is the owner here instead, but I'll ping both just in case.

HUD issues should be fixed now.