Just installed Cura 4.2 (upgraded from 4.1 where this was working) and found out that the Start G-Code is not used as a start any more.
The following lines had been set BEFORE my custom start code, which are heating the bed and the hotend, which is kind of bad doing so before the Start G-Code...
Lines in sliced file before "Start G-Code" :
;Filament used: 1.10656m
;Layer height: 0.12
;Generated with Cura_SteamEngine 4.2.0