PTS underflows are caused by the muxrate being set to low. The default for the TS profile is ATSC which is 19.2Mbps. If your video bitrate exceeds that at all, even for a short spike, you'll get that error.
For disk based files the muxrate doesn’t actually matter so if you run into this a lot just edit the profile and set it to something high like 50 and you'll never see it again.