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):

[![enter image description here][1]][1]

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.


  [1]: https://i.sstatic.net/A1dhf.png