Hi Xue,
I will start my answer by declaring myself as an ischial bursa sceptic.
The ischial bursa is supposed to be exactly where you describe (between glut max and the ischial tuberosity), however the conjoint hamstring tendon and the sacrotuberous ligament are on the ischial tuberosity so the bursa is actually between the g-max and these structures.
Rather than a bursa I just think of this as a plane of minor friction and when we inject it we are simply injecting the surface of the hamstring origin. I do sometimes see Doppler signal in this area suggesting localised inflammation. The reason I am a “bursa” sceptic is when we do ultrasound guided injections into this plane the fluid simply flows away and ends up sitting between G-max and quadrates femoris in the same potential plane as the sciatic nerve. In other words it acts as a plane rather than a bursa with defined margins. Despite this these injections do seem to provide good relief.
If your cystic lesions are in the subcutaneous space superficial to the G-max this is certainly not the correct location for the bursa,
In this space I would consider fat necrosis, injection site injury, sebaceous cyst, Morel lavallee lesion.
Do you have any images?
Steve