When using Layer Style: Stroke (Position: Inside), any new freehand brush dab on the layer creates strokes with severe artifacts, and those artifacts are kept in the saved files as well. By simply toggling the "Fx" button in Layer panel, the artifacts will disappear.
Created attachment 128862 [details] Artifacts of Layer Style: Stroke (Inside)
Created attachment 128864 [details] Artifacts of Layer Style: Stroke (Inside) disappeared after toggling Fx button
Created attachment 128881 [details] Stroke-Inside-Artifacts I can confirm this for the 4.3.0 beta-1 and the 4.2.9 appimages. However, I don't see that they are also in a saved .kra file. The artifacts don't happen on horizontal or vertical lines but do happen on diagonal lines and the outer edges of curves. They have the form of a step with an exponential decay. This does not happen with Stroke Outside. However, if you toggle off FX for Stroke Outside, there is a residual artifact that does not go away if you toggle layer visibility. It is not Saved in the .kra file. You can get rid it by removing FX for the layer.
Setting to Confirmed
Additionally, when Stroke's Size is set to 2px and smaller, it will also introduce severe artifact around the layer's image.
The bug has been fixed in commit a37cbcec401cab859c240037e3f377e12dbbd757 The fix has been released in 4.3.0
Thank you Dmitry! :D