Challenger's strange propellants

When I watch Challenger’s x-ray, I found some strange propellant models, looks just like badly made models or bugged.
Then I searched some image and video and found it’s real. Seems used for hesh.

My question is, why such a strange design.

What’s strange about it? It’s a normal design for a bag charge.
They’re not perfectly semi-circular like they look in War Thunder. It’s a non-rigid cloth bag stuffed with cord propellant (like a bag of spaghetti). Since they’re not rigid-cased, they are sort of malleable and of a size that you can stuff two in a single 120mm charge bin - which is why they’re modelled as semi-circles in War Thunder, occupying half of a single circular charge bin.

So, for hesh, it reduced charge. and made it a non-rigid cloth bag so can be stuffed two in a single 120mm charge bin to save space, which a rigid shell can’t do.
and it’s a way to avoid loader choosing the wrong propellant I guess

The SABOT rounds kind of needs a solid cased charge to engage properly with the cup the projectile sits in, but true that it uses a more powerful charge than the HESH round so is physically larger.
Bag charges are normal in the employment of non-fixed (i.e. 2+ parts) ammunition.