Temple of Febe Malicious Shadows loose immobilization

[Schyloe.2953]

I thought I was going insane for a long while when progging the CM.
As Druid it's my main job to immob Shadows.
I've noticed/felt that my immobilization was not working on them some how, even though I have around 16s of Immob in total, and I swear I hit ever single skill, it will only be Immob for around 6 or 8 seconds. 
Video, immob disappears randomly at 10s mark despite there being quite alot of immob on it.
This video confirmed my suspicions. 
Ty btw to the Void Lounge Discord for creating the discussion around it.