I spoke to the GFX team and was told that not all devices well support 16 bit and therefore it would need to check and convert down depending on the support.
The Area lights LUT does this: engine/webgl-graphics-device.js at b035de78c8e6fb7a34e560f83b3a55d697e173e6 · playcanvas/engine · GitHub
Another possibility is to use texture format PIXELFORMAT_111110F which is WebGL2 only but means you can upload a standard 8bit PNG but just change the texture format.