We are sorry for the trouble!
What you can do to improve quality of the compressed result is use Straight alpha
instead of Premultiply alpha
workflow, as described here.
Unfortunately, Unity 2019.3 does not offer DXT4
compression which would be the correct setting for premultiplied alpha textures.
Note that compression artifacts will always be around in a slight way, since block compression (which mostly operates on 4x4 texel blocks) is not too well suited for Sprite art with abrupt strong color or opacity changes.