System and IINA version:

• macOS 13.6.3
• IINA 1.3.4

Expected behavior:
When the setting Ignore ASS styles is not enabled style overrides are not applied.

Actual behavior:
When the setting Ignore ASS styles is not enabled IINA sets the mpv sub-ass-override option to yes causing style overrides to be applied. From the mpv manual entry for the sub-ass-override option:

--sub-ass-override=<yes|no|force|scale|strip>
Control whether user style overrides should be applied. Note that all of these overrides try to be somewhat smart about figuring out whether or not a subtitle is considered a "sign".

• no: Render subtitles as specified by the subtitle scripts, without overrides.
• yes: Apply all the --sub-ass-* style override options. Changing the default for any of these options can lead to incorrect subtitle rendering (default).
• force: Like yes, but also force all --sub-* options. Can break rendering easily.
• scale: Like yes, but also apply --sub-scale.
• strip: Radically strip all ASS tags and styles from the subtitle. This is equivalent to the old --no-ass / --no-sub-ass options.

This also controls some bitmap subtitle overrides, as well as HTML tags in formats like SRT, despite the name of the option.

When the setting Ignore ASS styles is not enabled IINA should be setting sub-ass-override to no.

Steps to reproduce:

• Start IINA
• Click on Settings… under the IINA menu
• The settings panel appears
• On the left side of the panel click on Subtitle
• In the ASS Subtitles section uncheck the Ignore ASS styles setting
• Move the thumb of the Override level slider all the way to the left
• On the left side of the panel click on Advanced
• Slide the Enable advanced settings toggle button to be on (blue)
• In the Additional mpv options section click on +
• A new entry appears in the table
• Double click on name and replace it with sub-ass-style-overrides
• Double click on value and replace it with FontName=Wingdings
• Confirm the panel looks like the screenshots below
• Restart IINA to activate the new settings
• Start a video playing with the attached subtitle file
• Even though Ignore ASS styles is not enabled the style override will be applied and the subtitles will look like the screenshot below

With Ignore ASS styles disabled the styles in the subtitle file were still overridden:

The subtitle file used for testing (with .txt added to make GitHub happy):
big-buck-bunny.ass.txt

• MPV does not have this problem.

How often does this happen?
Every time.