Manually Syncing Audio to Video goes out of Sync

If you use the media page to select a .mp4 video clip and then go to Waveform and select the corresponding audio .wav clip, you can line up the two using a suitable sync point (clapper).

If you then drag the video clip from the media pool to the timeline you see a link icon and a dot before the audio file name indicating the linkage. Strangely the file name is .mp4 and not .wav?
However if you look on the Fairlight page the file name is .wav? This is inconsistent.

More importantly when you play the clip on the timeline the audio and video components start in sync and then rapidly go out of sync.

To validate this, if you unlink the original .mp4 clip and then put it on the timeline it plays in sync with its original clip audio.
If you manually line up the .wav file under the clip and play it it all plays perfectly in sync.

What is the frame rate of your double system media?

It’s all 25fps.
Media is UHD converted to full HD using the optimised media option.