Disable smooth shading for this shape if you want flat shading.
Smooth shading doesn’t make any difference to the decimation, it’s only visual.
There is a small regression to decimation the latest Nomad version but most of the time it should work fine Decimation Issue - #4 by RogerRoger
Otherwise maybe delete UVs if you have it, the decimation algorithm needs to keep UV seams relatively intact otherwise: