Looks like there is in fact a problem: with the hashtag. This list of hashtags was posted by @4bcx :
Notice how the line is forced to the left, and the hashtag appears to the left of each word (same happens inside right-aligned Arabic sentences). It doesn't happen in the input (which i guess is under browser control); only when the toot is displayed.
#عرب #تنبيه #تحذير #ماستودون
Seems to do the same on Twitter.

Clarification: on Twitter # at line start forces the line to the left, like Mastodon, but the hashtag appears on the right of each tag, unlike Mastodon.