I first encountered this bug in the 30th anniversary version. I often retouch images using what I call a texture brush - a speckled pattern brush that is achieved by the Dual Brush option set to Multiply blending mode.
A bug was introduced in the 30th anniversary version, where the edges of the canvas were "protected", as in nothing was being applied to them, in relation to the brush size. Use a small brush, and only a tiny sliver of the edge cannot be touched. Use a very large brush, and you cannot have an effect over a larger area near the edge.
I uploaded a short video to youtube as a demonstration:
This bug has been reproduced by many other individuals using Dual Brush Clone Stamps on a variety of systems. The key factor seems to be the Dual Brush option. A regular brush (B) with Dual Brush works as it should, but Clone Stamp doesn't.
Windows 10, TR1950X, 64GB if that is of any concern. Please no "have you tried turning it off and on again" -tier answers.
This problem does not exist in versions prior to the 30th anniversary version.