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Previously, up until mid-December at least, all played files were stored in the playlist until you manually cleared them. Now it is reset every time you play a file and only the last file played is displayed.

This is vague and unclear. Works completely fine with RC4 and RC5 here.

It eludes me what you find either vague or unclear but I have explained the 2 sentences in excruciating detail below.

It may be helpful if I add that this behaviour occurs on 2 PCs both running Ubuntu 17.10 in Xorg display mode.

I'm assuming, "Previously, up until mid-December at least,..." is clear.
"...all played files were stored in the playlist..." means once I watched/played a movie its name would appear in a window titled Playlist, which is selected in the left hand side pane.
"...until you manually cleared them." means each title I played was appended to the bottom of the list in the Playlist window.
"Now it is reset every time you play a file ..." means that at some point precedent to completing the current title (as in watching it) the Playlist list is erased, ie. it is empty.
"...and only the last file played is displayed." means that subsequent to the completion of the last title watched the Playlist only contains the name of the last title watched.

Actually i have erred in this post. This problem is an extension to the problem of not being able to select a file to play from a network location which causes one to play a file from the OS file manager. What is actually happening is that for each title played a new VLC window is opened which has a blank playlist, even if those files were selected as a group in 1 operation.