The reason for this is that I stumbled upon the PNG Optimization Level-parameter which I set to "Level_1_(recommended)" ("recommended" sounds like a good choice at first sight):

Since some of the resulting images remain RGB and some a converted to palette, my working hypothesis is that if the parameter is set to a value >0 FME tries to estimate if a conversion to palette leads to a significant reduction in file size. So, leaving this to "No_optimization" always results in RGB interpretation.

The reason for this is that I stumbled upon the PNG Optimization Level-parameter which I set to "Level_1_(recommended)" ("recommended" sounds like a good choice at first sight):

Since some of the resulting images remain RGB and some a converted to palette, my working hypothesis is that if the parameter is set to a value >0 FME tries to estimate if a conversion to palette leads to a significant reduction in file size. So, leaving this to "No_optimization" always results in RGB interpretation provided the source is RGB.