Time to seek to is not properly handled

[Zakret]

It seems that the thumbnail taken does not come from a set time by the user, but from the nearest quotient n*x greater than that time (I noticed that 'x' is different for the same video encoded differently). If this is intended behavior, I think that the user should be informed about it (e.g. in the --help option).
I generated 20s video:

x265 variant:

Stream #0:00x1: Video: hevc (Main) (hvc1 / 0x31637668), yuv420p(tv, smpte170m), 640x480 [SAR 1:1 DAR 4:3], 278 kb/s, 29.97 fps, 29.97 tbr, 30k tbn (default)

x264 variant:

Stream #0:00x1: Video: h264 (High) (avc1 / 0x31637661), yuv420p(tv, smpte170m, progressive), 640x480 [SAR 1:1 DAR 4:3], 333 kb/s, 29.97 fps, 29.97 tbr, 30k tbn (default)

Expected result:

ffmpeg -ss 00:00:10 -i ./time_test_x265.mp4 -frames:v 1 -q:v 2 ./time_test_x265_ffmpeg_10sec.jpg

ffmpeg -ss 00:00:10 -i ./time_test_x264.mp4 -frames:v 1 -q:v 2 ./time_test_x264_ffmpeg_10sec.jpg

Results (presented side by side: x265/x265):

ffmpegthumbnailer -t 00:00:01 -i ./time_test_x265.mp4 -o ./time_test_x265_ffmpethumbnailer_1sec.jpg
ffmpegthumbnailer -t 00:00:01 -i ./time_test_x264.mp4 -o ./time_test_x264_ffmpethumbnailer_1sec.jpg

ffmpegthumbnailer -t 00:00:04 -i ./time_test_x265.mp4 -o ./time_test_x265_ffmpethumbnailer_4sec.jpg
ffmpegthumbnailer -t 00:00:04 -i ./time_test_x264.mp4 -o ./time_test_x264_ffmpethumbnailer_4sec.jpg

ffmpegthumbnailer -t 00:00:06 -i ./time_test_x265.mp4 -o ./time_test_x265_ffmpethumbnailer_6sec.jpg
ffmpegthumbnailer -t 00:00:06 -i ./time_test_x264.mp4 -o ./time_test_x264_ffmpethumbnailer_6sec.jpg

ffmpegthumbnailer -t 00:00:18 -i ./time_test_x265.mp4 -o ./time_test_x265_ffmpethumbnailer_18sec.jpg
Seeking in video failed Operation not permitted, will use first frame.
ffmpegthumbnailer -t 00:00:18 -i ./time_test_x264.mp4 -o ./time_test_x264_ffmpethumbnailer_18sec.jpg