Encoding H265 and AV1 give different framerate from ffmpeg

I’ve written code to do encoding with NVENC. The same code runs for both H265 and AV1, i.e. frameRateNum and frameRateDen are the same (60 and 1 respectively), for 60 fps. I’m writing out 60 frame GOPs, i.e. 1 second of video per file.

When I ffprobe each GOP, the H265 encoded files give:

Video: hevc (Main 10), yuv420p10le(tv), 1280x720, 60 fps, 60 tbr, 1200k tbn

but the AV1 encoded files give:

Video: av1 (Main), yuv420p10le(tv), 1280x720 [SAR 1:1 DAR 16:9], 25 fps, 25 tbr, 1200k tbn

For some reason the AV1 is showing as 25 fps when I explicitly set it to 60. What mistake have I made? Setting enableTimingInfo in the av1Config structure doesn’t make any difference and I can’t find any other parameter. Is this just a quirk of ffmpeg or is 25 fps somehow baked into the stream?

Note: When I count packets with ffprobe, using the following I get 60 in both cases. I suppose this is an issue with ffmpeg/ffprobe and not my encoding?

ffprobe -v error -select_streams v:0 -count_packets -show_entries stream=nb_read_packets -of csv=p=0 “1.av1”