System and IINA version:

• macOS Mojave 10.14.6 (18G103)
• IINA 1.3.0 Build 132 mpv 0.34.1

Steps to reproduce:
— while a video file with at least one subtitle track(s) is open
— select some subtitle track as primary
— try to select the same subtitle track as a second subtitle

Expected behaviour:
either
— the same subtitle displays now in both primary and secondary position
or
— subtitle position switched from primary to secondary position

Actual behaviour:
— there is no change, no reaction on the secondary subtitle selection whatsoever: primary subtitle is still active, and secondary subtitle is still empty.

How often does this happen?
It's a stable problem, it happens always.