Backward compatibility is normally good. Recently, improvements necessitated changes where some effects or aspect ratios from past versions may not render correctly.
Thus it's wise to export with the same, or a close version in time, to the one that edited and saved the project.
VP is in a constant state of development. Bugs are quickly addressed and can be reported by clicking the HELP menu chevron at the right-top of the VP window.
You can always install the latest unlicensed version, to see if you like it. Features will be limited after a period of time. Retain your old installer, or find it in this path...
"C:\Program Files (x86)\NCH Software\VideoPad"
No other reports of missing subtitles have been posted AFAIK.