some chinese simplified characters in subtitles are displayed as a box only

this issue occurs only under ios not under ms windows and not under android. at present I know that at least 3 characters are displayed wrong, but only simplified characters. That means whenever a subtitle contains

1. 远 like in 永远 then only 永□ appears.

2. 虑 like in 考虑 then only 考□ appears.

3. 敌 like in 敌人 then only □人 appears.

If these character are replaced by the traditional chinese one, the error disappears.