Unfortunately, I was not able to try anymore yesterday as Plex was being used quite a bit so I could (did not 
) want to interrupt.
However, I tried a few more times this morning, and knowing what I know now I see what happened.
Indeed, repeat one was interpreted as repeat once and if I select this item, the specific current clip gets repeated. This explains why some clips actually “worked” yesterday.
Selecting repeat repeats the entire playlist if I forward to the last few seconds of the last clip. If I do not have either of those selected and try the same, the playlist interface is once again shown instead.
So, this appears to be user error. Ideally, some text should be shown alongside the repeat options and/or selected option to let the user know how repeat and repeat one (not to be confused with repeat once are used).
In the meantime, we can let this forum thread be the documentation for others who may run into the same issue.
Thank you once again @ljunkie for taking the time here and getting me properly situated.
Since you have the hot hand here , please consider assisting with my two other outstanding items of note here with Plex:
Otherwise, thank you again for your assistance. 
