Not sure why this is happening, I guess you could disable backface culling.

Also, do you by chance have advanced OpenGL enabled? I've had related problems with different mods, where some faces of a texture would disappear.

Can't reproduce this. Can you confirm this is a regular Mining Turtle (not your own upgrade), and that you don't have any resource packs installed that change the model of the pick?

Normal upgrade with no resource packs in a dev environment. My computer is ancient so that may be something to do with it.

Is there any upgrade attached to the other side of the turtle?

In this case yes, but I can reproduce without one.

Can you create a new superflat world, select a mining turtle from the creative menu, place it at your feet facing north, and tell me the co-ordinates of the turtle

I'm not currently on my home computer - I'll have a go this evening. Sorry.

No worries.

Alright. I can't replicate this. Closing, but will re-open if anyone else reports it.

Screenshot with debug view.

I tried a pickaxe in an item frame but that renders fine. I tried several other items(pork and steak) but they rendered correctly.