Everything “looks” correct, Nathi, and I have not been able to replicate the first issue – can you email me the file so I can have a closer look? pcaggegi@autotroph.com
As for the join function – I completely understand – this has not yet been fixed in Grease Pencil 3.0, and I HATE it. it does NOT join two strokes together; it creates a NEW stroke and places it over the gap. This is not what I want, especially if I wish to replace a material. I’m watching the bug reporting on this and perhaps I have not been specific enough on my request to effect any change. 4.2.x and Grease Pencil 2.0 – this will give you no issues.