I’m pretty sure browsers will automatically interpret Arabic text as right-to-left, even without an explicit dir=rtl attribute.

According to my understanding actually, it's not because the default value of dir attribute is ltr even if the browser's language is set to Arabic, the browser will set the correct direction automatically if the default value of dir is auto see this example

also, the issue it is becoming very hard to read when mixing Arabic and English here's how we read it before fix

the blue arrows (number 3) it's difficult and uncomfortable to read

here's how we read it after fix

this is the comfortable and correct way we read Arabic mixed with English.