Operating system: Windows 10 Pro version 1909 build 18363.476
Graphics card: NVIDIA GeForce 940MX
Short description of error
Properties > Output Properties > Output > File Format is set to BMP and Color is set to BW. Output files are BW, but file-size confirms they are actually 24-bit BMPs with identical content in each color channel. Not a huge problem, because I think almost any software that can read BMPs can read the 24-bit version. But it does triple the file size. And Blender can input 8-bit BMPs, so this is just an output issue.
Exact steps for others to reproduce the error
Minimal example blend file attached.