I did notice the new web UI myself a day or 2 ago, on both my desktop browser and mobile browser (I don't use the mobile app).
The main change I see on the mobile is they moved the item selection options -- like Mark Spam, Move to Folder -- out from a hidden tap, to a visible floating footer bar. Personally, either is fine for me.
But...
Bug 1: Formerly, attached calendar invites and all contained details would appear within the body of the email. Not so now. I just see an attachment "winmail.dat" (calendar invite sent from someone with MS Outlook I presume).
Bug 2: I'm one of those weird users that Control-clicks select messages to read in new browser tabs, a handful at a time. Problem: After opening and reading these message, they are no longer marked as Read on the main message list page (they used to be), even if I refresh/reload the mailbox or browser page. I have to then re-open each message in the main list again, just to de-bold it as Read.
(Tedious and frustrating.)
Bug 3: I have my spam delivered to my inbox, with {SPAM xx.xx ###} pre-pended to the subject line. If it's NOT spam, I take 2 steps: Move to Spam + click Not Spam, and the message then gets moved back to Inbox and the pre-pended "SPAM..." tag is properly removed. Now, it's not removed when the message moves back.
I hope FM staff sees this post and considers implementing bugfixes.