I encountered an issue when I started with a shapefile line feature which held on to it's format attribute SHAPE_GEOMETRY as being an arc even though I had used the buffer it and area on area overlay transformers during the workbench.
I took me an age to figure out why it wasn't writing out as area features despite everything else within FME told me they were in fact areas: inspector showing the features as areas, was able to calculate the areas of each feature, my features were definitely areas! Eventually I found the SHAPE_GEOMETRY format attribute (is this new?) and found this doesn't update throughout the workbench automatically. Once I changed this using an attribute_creator to shp_polygon the problem was fixed.
I'm not sure why this has just started happening (I've never had this issue in the past) but if Safe if you could update this in one way or another that would be much appreciated.

