It seems you are right. Saving as the original playlist name seems to delete the contents. However if you save as another name it doesn't. I opened some old playlists & added files to them & saved as another name. When I re-opened them (after opening others), the playlists were fine & had the additions. So I guess the motto is that you "must" save using a new playlist filename each occasion. That means you'll need to delete the older ones if you deem them to be no longer useful. At least that aspect works.

Perhaps you can make a New Feature Request (in that section of the forum) to make improvements for the playlist such as being able to append an old playlist without having to re-save it under a different name.

BTW I saved my m3u playlists to root level of another drive so it was easy to find & access them (drive F.) In the tests I did above, I added/enqueued files to the playlist from all parts of the computer, drive C & G, & several locations on each just to be sure.