First of all according to the child theme architecture if you call a file with same name it will load from your child theme. the classic theme has a file called theme_rtl.css
that is why it load from child theme and all rtl related css is gone. I have tested and here is the final solution
1. remove theme_rtl.css from child theme
2. create a file 1.7.7.5\themes\chaild\assets\css\custom_rtl.css and put all modification then it will looks like this
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Thank you