What you want IINA to do:

I would like IINA "load external subtitles..." to open the directory containing the current video file

Maybe even auto-detect subtitle files by extension or by subfolder(subs folder for example).

What IINA does currently:
Opens the last opened directory opened with this action. For me it just means it opens a different movie folder, sometimes in the "subs" directory of that directory. Before I knew this was the behavior I opened the english subtitles in the to only have loaded the subs for a different video.

Why you think this should be added:
Less annoying and confusion.

Examples of other projects that have something similar:
VLC