This is something I have already talked in the past: mixing a transmitting material with transparency is a can of worm you don't wont to open.
Transparency makes the object (and the associate volume) literally to not exist. LuxCore can maintain consistency while tracing a single ray: if the ray intersect a sphere, the sphere is solid or transparent both for the entry point and the exit.
However it is impossible to maintain consistency over a chain of rays (i.e. a path). In your case, the path hit the sphere surface and a shadow ray is traced. The material can transmit so the shadow ray can be traced trough the back of the sphere surface. However the shadow ray has a new random value to decide if the sphere is solid or not.
The sphere can become suddenly transparent for the shadow ray because of the new random value so it lighted by the light source. This causes also a huge mess with volumes because you entered in a volume but you never exited
Short version: I mark this as "wontfix" (more correctly it is a "cantfix").
You can probably show a big warning in BlendLuxCore for all transmitting materials if the transparency socket is used. However can be hard to say for recursive materials like Mix or GlossyCoatting.
Transmitting materials are: