"The end of 立ち読み (Tachiyomi) is the beginning of みほん (Mihon)"
Credit to LinkCable, the icon designer, for this poetic quote.
What's New?
Well, nothing, except you now you need Android 8+ to install...
It's a fork, done by a previous Tachiyomi developer and maybe a TachiyomiSY developer too. So it's partly a publishing team change, as the main developers are different.
Since Mihon is exactly the same as Tachiyomi, besides changed branding and requiring Android 8+, it's also a rebrand.
Edit: It's also functionally exactly the same software.