@Whatamidoing (WMF) @PPelberg (WMF) Hello, try to reply in Visual mode. Write 99 add add subscript and small formating. Then write 66 and add superscript and small formating. Then publish post and there is <meta /> tag, see this diff.
Topic on Talk:Talk pages project/Replying/Flow
I haven't been able to reproduce by following these steps. Does it happen for you consistently when you do this?
It's apparently not an isolated issue, here's another diff with a <meta /> tag: https://bn.wikipedia.org/?diff=4787950 (I just ran into it by chance, while looking at the largest edits made with DiscussionTools)
Now, I cannot replicate it too (see Patchdemo). It happened only one time at Czech Wikipedia.
I wonder whether there's something with copying and pasting – a bit of strange formatting getting lost? The bnwiki diff has 'blank' lines.
I am not sure if I copied some text originally…
Well, it's in Phab now, and maybe one of the devs will have an idea about what's going on. 
Thank you so much for reporting this. I'm pretty sure that it would have been entirely missed if you hadn't said something.