I tried with this parameter. No, it didn't fix the situation.
Well, okay, not critical.
By the way, if I put the screenshotformat = png parameter in start.ini, then all the same screenshots are created in jpg format.
Because this is the comment in my example start.ini:
;screenshotformat values supported: bmp, tga, jpg, default is jpg if not specified
png is not among the supported values, and if it doesn't match any of the supported, it will default to jpg type
