Not sure if this issue affects other applications on OS X, but it’s surely reproducible in VLC.

A solution is proposed in the VLC link above, but, in my opinion, mpv is more appealing, so I shall share the updated Apple Script for mpv:

1
2
3
4
5
6
7
8
9
10
11
on run
	do shell script "open -n /Applications/mpv.app"
	tell application "mpv" to activate
end run

on open theFiles
	repeat with theFile in theFiles
		do shell script "open -na /Applications/mpv.app " & quote & (POSIX path of theFile) & quote
	end repeat
	tell application "mpv" to activate
end open

Just save it as mmpv in Application format in Applicatoins folder, and use it as the default players for all videos, then all the trouble goes away.