Font Size Mismatch When Using Fallback Fonts in FFmpeg ASS Subtitles - Stack Overflow

I’m facing an issue with font size mismatch in ASS subtitles rendered by FFmpeg when combining English text with Arabic or Persian characters.

Steps to Reproduce:

1.Subtitle File: Here’s an example of the ASS subtitle file:

[Script Info]
Title: Generated ASS
ScriptType: v4.00+
WrapStyle: 2
ScaledBorderAndShadow: yes
YCbCr Matrix: none
PlayResX: 1920
PlayResY: 1080

[V4+ Styles]
Format: Name, Fontname, Fontsize, PrimaryColour, SecondaryColour, OutlineColour, BackColour, Bold, Italic, Underline, StrikeOut, ScaleX, ScaleY, Spacing, Angle, BorderStyle, Outline, Shadow, Alignment, MarginL, MarginR, MarginV, Encoding
Style: Default, ABeeZee, 40, &H00FFFFFF, &H000000FF, &H00000000, &H00000000, -1, 0, 0, 0, 100, 100, 0, 0, 1, 1, 0, 2, 10, 10, 10, 1

[Events]
Format: Layer, Start, End, Style, Name, MarginL, MarginR, MarginV, Effect, Text
Dialogue: 0,0:00:01.00,0:00:05.00,Default,,0,0,0,,Hello! This is English text.
Dialogue: 0,0:00:05.00,0:00:10.00,Default,,0,0,0,,سلام! This is mixed Arabic/English text.

2. Command Used:

ffmpeg -i input.mp4 -vf "ass=subtitle.ass" output.mp4

3. Screenshots: Below are two screenshots of the issue:

• Text rendered with the main font (ABeeZee) is correct.

• Text rendered with the fallback font (Cinema) appears larger/smaller in size.

5. Debug Logs:

[Parsed_ass_0 @ 0xb4000079f04f8210] libass API version: 0x1701000
[Parsed_ass_0 @ 0xb4000079f04f8210] 
[Parsed_ass_0 @ 0xb4000079f04f8210] libass source: commit: 0.17.1-0-ge8ad72accd3a84268275a9385beb701c9284e5b3
[Parsed_ass_0 @ 0xb4000079f04f8210] 
[Parsed_ass_0 @ 0xb4000079f04f8210] Shaper: FriBidi 1.0.13 (SIMPLE) HarfBuzz-ng 8.0.1 (COMPLEX)
[Parsed_ass_0 @ 0xb4000079f04f8210] 
[Parsed_ass_0 @ 0xb4000079f04f8210] Using font provider fontconfig
[Parsed_ass_0 @ 0xb4000079f04f8210] 
[Parsed_ass_0 @ 0xb4000079f04f8210] Added subtitle file: '/data/***/ass_file.ass' (4 styles, 29 events)
[Parsed_ass_0 @ 0xb4000079f04f8210] 
[Parsed_ass_0 @ 0xb4000079f04f8210] fontselect: (ABeeZee, 400, 0) -> /data/***/ABeeZee_regular.ttf, 0, ABeeZee-Regular
[Parsed_ass_0 @ 0xb4000079f04f8210] 
[Parsed_ass_0 @ 0xb4000079f04f8210] Glyph 0x633 not found, selecting one more font for (ABeeZee, 400, 0)
[Parsed_ass_0 @ 0xb4000079f04f8210] 
[Parsed_ass_0 @ 0xb4000079f04f8210] fontselect: (ABeeZee, 400, 0) -> /data/***/Cinema.ttf, 0, Cinema

Question:

How can I ensure that text rendered with fallback fonts has the same size as the main font? Are there specific settings in FFmpeg or the ASS subtitle format that can address this issue? Alternatively, is there a way to manually adjust the scaling of fallback fonts in FFmpeg?