ffprobe – ffmpeg segmentation without re-encoding drops frames

I have an input file data. When I run the command ffprobe -i data -show_streams -count_frames -select_streams v | grep nb_read_frames, I count 39201 frames.

Now, I try to segment the video using command ffmpeg -i data -c copy -f segment -segment_time 120 -reset_timestamps 1 data_part_%d.mkv. This command generate 11 segments. To count the total number of frames across all segments, I ran the following loop: for i in {0..10}; do ffprobe -i data_part_$i.mkv -show_streams -count_frames -select_streams v | grep nb_read_frames | cut -d '=' -f 2; done | awk '{s+=$1} END {print s}'. This sums up to 36,684 frames, which is 2,517 frames less than the original count.

Some additional observations I made (not sure if they are relevant):

1. I found a segment where ffprobe reported a duration of 2 minutes, but when I inspected the timestamps of the frames of that segment, the last timestamp was actually at 63 seconds.
2. When I play the segment with VLC, it is actually 2 minutes long. The last 57 seconds are all black frames. The black frames are expected: the data file also has a lot of consecutive black frames.

Why is there a discrepancy in the frame count between the original video and the segmented parts, and how can I avoid losing frames while segmenting without re-encoding?