Grease Pencil Developer Antonio Vazquez has authored a new patch that greatly improves Grease Pencil's stroke joining operation.
— antonioya (@antonioya_blend) September 3, 2021
More specifically, this new patch allows for much better results when using automerge. Previously, joining strokes with vastly different thicknesses via auto-merge yielded less than optimal results. This has been solved by adding a smooth to the joining operation.
This improvement in joining consistency and behaviour is not confined to situations where strokes greatly vary in size, but also cover all use-cases including same-size strokes, as seen in the video by Antonio above.