SOLVED!! on Ubuntu 20.04 LTS & PMS 1.22.0.4163
The external srt files needs to contain at the end of the srt file two(2) blank lines after the last line of the last subtitle. Nothing more, nothing less. These blank lines are inserted automatically by opening the srt file with the program Subtitle Edit (eg v358). Just open the file and save it. Check your srt file with notepad or any other editor and you will notice two blank lines have been inserted at the end. Since doing this systematically for my srt files I do not encounter subtitles that disappear. In case the problems occurs, check the srt file, if the two blank lines are present at the end, if not insert two blank lines.
