Don't get me wrong, generally I like MEP (2017 Plus).  But it can be infuriatingly cranky.

A facility I use a lot is multi-cam. I like the flexibility to substitute one shot for another and drag cuts along the timeline. [It has some idiosyncrasies you have to work round, the main ones being that the video sources have to be video not stills, and that all of them, and the audio, have to be contiguous - no gaps. And if you squeeze or expand your master sound track to match the video, audio replay during editing will likely fail at some point along the timeline; I cure this by exporting audio to e.g. Audacity and re-saving it in order to re-establish a correctly timed data structure in the WAV file.]

However.... Yesterday I had done some hours of multi-cam work on a two hour timeline with a hundred plus cuts and was happily working through doing fine adjustments.  Then I thought I'd render an output to see if the whole thing "felt right". When that completed, I saved the project and shut down the computer for the night.

This morning any attempt to do what I had successfully been doing yesterday fails. Specifically the following - please refer to the diagram which shows an example Track 1 situation, with a cut between Camera A and Camera B.

Selecting the camera B clip, I split it into two, and select the smaller chunk.

Now I go into multi-cam mode, and click on the Camera C thumbnail. Normally Camera C's feed would now fill the space of the little B chunk.  But this morning the whole of track 1 becomes C, as shown.

There is nothing worse for the user (and his/her blood pressure) than software which behaves differently after quite legitimate user actions. To be fair and disclose everything, I had added a timecode to the project last night before the render (which of course took long enough for my autosaves to completely overwrite any possible "good" project copies).  However I have recreated multiple test projects this morning using the same data sources, with and without inserting a timecode, and of course they all follow Murphy's Law and work perfectly.  But not my actual work project which (sob) is lying on its back with its legs in the air.....

I write on the off-chance that someone else has experienced, and solved, this particular "effect".  Probably this is not a factor but the HD project had four 1080p/50 sources plus a separate WAV audio source; the render mentioned was down to 720p.

Cheers  - Mike -