In one part of the image, Nebula is trail while stars are round ! [Deep Sky] Processing techniques · Ali Alhawas · ... · 4 · 221 · 1

Hi,

Got 109 images x120" exposure..When stacked, and in the upper left part you can see that the nebula is trail while stars are good !! means its not guiding issue ! 
I tried the following:
Stacked three groups of images, 36 each to isolate the affected ones >> all gives the same bad result.Tried both APP and DSS >> same result.Tried to stack the lights only w/o any calibration frames>> same result.I checked every single frame >> they are good and stars are round (except a small amount of sensor tilt) which I accepted.Revised PHD2 reports>> all good
NB: Due to the target is a Dark nebula, Its too hard to see any nebula in a single 120" image even with strong stretching , all you can see are round stars and a very faint dark nebula which you can't decide if its trail or not.
 Gears:Askar ACL200 f4, I used many times and gives a good result
ASI 2600mc pro, Zero temp. sensor, with camera anti-dew heater turned ON
AM5 mount
ASI 290mm mini with WO 50mm scope as a guider.
Ambient temp. was 18c and humidity is around 30% 
This is the first time I faced this kind of problem, ONLY one part of the image is affected !!
Any suggestions?
Here you can see:the stacked image (Pixinsight ABE and stretched) with the affected area :
https://www.dropbox....t=h7kbthen&dl=0
And here is a 120" single image (Pixinsight ABE and stretched)
https://www.dropbox....t=e0xr1bf1&dl=0

Yeah, Jupiter…

2 fields away should be just right. From the edge of the frame, obvs.