Hi, I am having similar issues. Basically, I made a blurred rectangle about a year ago, and now I want to have multiple rectangles with different blurs (to represent laser light intensity for context). When I duplicate the original shape, the blur remains how I want it, but if I change the blur amount then I get some cut off. I have tried your suggestion of looking at blend mode, and also tried to change the filter editor selection size as someone else suggested, but nothing works. I've linked an example, where I have duplicated the shape, changed the blur, then changed the blur back to the original value, and the cut off remains. Any help would be appreciated as this is driving me mad...

https://www.dropbox.com/s/6tsaf9eqq9f1mp7/Blur_cut_off.svg?dl=0