@bigasa It's very hard to diagnose just based on two small screenshot regions without knowing your scene. A quick guess is that you are perhaps having an almost fully transparent attachment (e.g. alpha 0.05) enabled in the problematic animation which is casting shadows (in conjunction with a low shadow-casting alpha threshold value).
If that's not the case, could you please send us a minimal Unity project that still shows this issue? You can send it as a zip file to contact@esotericsoftware.com, briefly mentioning this forum thread URL so that we know the context. Then we can have a look at what's going wrong.