I am having the same zig-zag-ed loss values in one of my works.
In that work, I am dealing with multi-label (multiple hot encoder) of size 600.
I think this rather large multi-hot binary output is the reason for this zig-zag-ness.
So, the large variations in the input/output could be the reason for this phenomenon. I would not worry about it as long as the model is achieving convergence.