SUMMARY
The SRT files produced either during the edit, or upon Project>Subtitles>Export Subtitle are storing the timecodes in H:MM:SS,FFF instead of HH:MM:SS,FFF. It's missing the leading zero.

For example:

1
0:00:12,840 --> 0:00:14,040
Hello everyone,

Should be:

1
00:00:12,840 --> 00:00:14,040
Hello everyone,

This can cause issues with "incorrect encoding" when importing these subtitles into other software (for example KDE Subtitle Composer https://subtitlecomposer.kde.org/)


STEPS TO REPRODUCE
1. Type a subtitle into the timeline of a project
2. Either look at the autogenerated SRT or export subtitle from Kdenlive
3. Compare the file to another SRT, or import it into other software

OBSERVED RESULT
Format is 0:00:00,000 (H:MM:SS,FFF)
"Could not parse the subtitle file.
This may have been caused by usage of the wrong encoding."

EXPECTED RESULT
Format is 00:00:00,000 (HH:MM:SS,FFF)
It is imported successfully into other programs

SOFTWARE/OS VERSIONS
Windows: 
macOS: 
Linux/KDE Plasma: Ubuntu 18.04.4 LTS
(available in About System)
KDE Plasma Version: 
KDE Frameworks Version: 
Qt Version: 

ADDITIONAL INFORMATION
Converting the Kdenlive SRT (to SRT) with an online converter adds the leading 0s, which allows it to be imported/re-edited by other software.