I personally had never tried to save different bpc per layer into a single multilayer .exr on purpose, but I made a quick test and you are absolutely right. These images show the issue you mention
By default (saving through the main output panel) channels such as Depth get stored as Full float, even if Half float is the selected configuration, and I think it is like that by design.
The same file exported through the file output node stores depth as half-float, so I would consider it a bug. I don’t think it is related to this thread though. I think it is more related to the other one I opened no so long ago, that one is about the inconsistency in the metadata stored depending on which method you use to save the render (main output vs compositor), so I think it would be a better fit. But I would report it as a bug either way.