I’m guessing the reverser only gets applied when the UI language is set to Hebrew/Arabic/etc., this is not the case here. RTL text can appear in content even when the UI language is LTR.
As Udi mentioned, it’s often preferable to use -inline-start/end instead of -left/right in style sheets, and not using an error-prone reverser. This would work correctly both for embedded RTL (in post contents) as well as layout RTL (based on selected UI language), with only a single style sheet. You might want to consider migrating to that and retiring rtlcss. But of course, you don’t have to, if there isn’t a real issue to solve.