As your input file report a strange frame rate value 1k fps coming from the tbs and tbr value (look here for their definition)
the encoder generate a different result, 16k tbn, 1k tbc (default)
So by calling :
ffmpeg -fflags +genpts -i 1.webm -r 24 1.mp4
You configure ffmpeg to generate new pts (a.k.a Presentation TimeStamp) for each frame and you set the target frame-rate to 24.
So your output mp4 file info (ffmpeg -i ....) change from
Stream #0:0(eng): Video: h264 (High) (avc1 / 0x31637661), yuv420p, 1280x720 [SAR 1:1 DAR 16:9], 4327 kb/s, 1000.09 fps, 1k tbr, 16k tbn, 2k tbc
to
Stream #0:0(eng): Video: h264 (High) (avc1 / 0x31637661), yuv420p, 1280x720 [SAR 1:1 DAR 16:9], 1670 kb/s, 24 fps, 24 tbr, 12288 tbn, 48 tbc