First. I’ll assume you are properly refocusing after zooming from 150mm to 600mm, and not just taking the 600mm shot after zooming without refocusing. Lens infrastructure is good, but not so goo that it will maintain a perfect focus when moving all those lens elements back and forth.
I’ll assume the top pair of images are at 150mm without and with a filter of some sort, and the bottom unlabeled pair are at 600mm. That being the case, first you need to zoom in on then 150mm images using Photoshop or some other editor to determine if the two 150mm images are equally clear (get to the same crop as you show in the 600mm shot). If the filter 150mm image is “blurry” when the unfiltered image is not, then the likely suspect is the filter. What type and what quality of filter are you using?
If the two 150mm images are equally clear when pixel-peeping, and you have ruled out possible user/environmental motion, then I would suggest the likely suspect is the loss of focus integrity at the longer focal length. I’m sure you know the autofocus “mechanics” are a function of both the lens and the body/sensor, and sometimes, a given lens/sensor combo is slightly out of focus-adjustment, thus the reason for the micro-adjusting capabilities built into all removable lens digital cameras. The adjustments can easily fix a focus issue in prime lenses where the lens elements are only being moved to focus, but in a zoom lens where the lens elements are also being moved thru the barrel to change focal length, it is impossible to micro-adjust every conceivable focal length, in this case between 150mm and 600mm. So micro-adjusting at 600mm to get a pin-sharp focus will likely yield less sharpness at other focal lengths. Try micro-adjusting the focus of the camera/lens combo at 600mm and then repeat the same test and see if the 150mm images are thrown out of focus.