Looking at the encoded bitstream in both imhex and my tools, it looks like the capture_time_distance in frame_info in the frame_header is always written as 0. This happens even with Y4M that specifies the frame rate, or with the --fps encoder option.
I was hoping to use this value in the code stream to determine the indicative frame rate (e.g. with just the codestream). Is there anything else that provides this information?
If not, it may require external information to properly do the multiplexing into ISOBMFF.