Parts of Flat Bottom Layers Not Being Supported

[brian_prefix 0]

Running Cura 4.2.1. I've been having a problem with flat bottom layers having sections that supports won't build under. It's happened to 3 or 4 different parts in the recent past. Attached are pics of back-to-back layers to show where the supports are building in relation to the flat build areas. As you can see parts of the surfaces are fully supported, and parts have no supports. View starts at layer 2 to remove the brim for clarity. Due to the nature of the part, I cannot upload a 3MF file or a STL, nor the GCODE. I am using the default profile for the DuPont CF Nylon & Ultimaker PVA, and have also sliced it with default Ultimaker Tough PLA and have had the same results.