Camera Lens Blur's "Repeat Edge Pixels" feature doesn't behave as expected along the left edge at small (≤.5 pixel) values. The attached project file shows this behavior, from frame 11 to 30. Above .5 pixels, the feature behaves as expected.

Adjusting with the Rotation and Aspect Ratio values can change the threshold at which this behavior disappears, but doesn't fix the issue.

Tested on an updated AE2020 (17.1.4, build 37), Win10 x64. Occurs at all bit depths and compositing modes I could think to try.

The "Attach a File" thing's being weird, so you can download the file here:

https://drive.google.com/file/d/1yjUz2T4rf8i7mFvpbr5f7JZjE0fOF1