I'm on the MIUI 10 Beta now, but I saw it before that too.
The strange thing is that it only happens the first time a message loads.
If I reload the page either by turning the phone to horizontal, then vertical again, it fixes itself. Same thing if I use volume buttons to change text size (up and down).
I have been running lots of apps (including one I've written myself), and never seen this behaviour anywhere but on AquaMail.