The armour model for Stug III ausf B (tier 4 German TD) is broken
You are probably referencing the PC v1.0 models.
Sorry my reply below is copy pasted from a similar thread.
WG suddenly changed all models and now stores them into several new formats. They provided zero advance warning or even guidance on how to solve them. You may blame wargaming for this, but for Andrew it is suddenly an enormous amount of work to solve not only one new format, but several from scratch.
Most of the new models For PCv1.0 were solved, but not all. If fixing them all finally proves too difficult or lengthy, maybe the old models will be available as an option. The problem with older models is that armor plates thicknesses may not reflect newer values anymore. The other side of this issue is that including all models for all platform and the previous versions as well will make the app too heavy for many of the older device.
We are sorry for the inconvenience, and we are suffering as much as you. Please be patient.