How to use VLC to convert MKV files to MP4 files without the video being shortened

Question

I'm attempting to use VLC to convert an MKV file (containing 1920 x 1080 video) to an MP4 file but I'm not having a great deal of success. The MKV file is a 6 minute video where most of the time, just a map is displayed and nothing is happening. At some stage, an aircraft symbol is displayed and every few seconds this moves by about 10 pixels. After converting to MP4, the video is speeded up and the new length of the MP4 video is around 20 seconds. It's as though all of the frames that are the same as the previous frame are thrown away.

I'm very new to VLC (version 3.0.2 running under Windows 10) and its settings but in the process of thrashing around, I've tried the following Profiles on the Convert dialog:

'Video for MPEG4 1080p TV/device' - throws away the frames where nothing changes and the remaining video is blurred. 'Video - H.264 + MP3 (MP4)' and 'Video - H.265 + MP3 (MP4)' - throws away the frames where nothing changes and the remaining video is as clear as the original.

I'm sure there must be an obvious setting somewhere but I can't spot it in VLC's setting, nor via an internet search.

I'd be grateful if someone could point the way and provide some advice.

Thank you.