In 18.10 / GNOME 3.30.1 Rhythmbox 3.4.2 now has this UI (same when maximised)
many menus and controls are absent, I don't have a pre-upgrade screenshot from 18.04 / <= GNOME 3.30.1 but this one from Ubuntu MATE 18.04 is how it looked in Ubuntu 18.04's GNOME:
Both Rhythmboxes have a "Speed Control" plugin installed, in the second screenshot it's between the shuffle button and the album art.
Perhaps it was broken in Rhythmbox 3.4.2 or GNOME 3.30.1, that's ok.
However, the other main feature of the second screenshot is the top application menus - File, Edit, View...
These are nowhere to be found in GNOME's layout for Rhythmbox 3.4.2 and these remove a lot of usability from Rhythmbox.
For example, the Play Queue in the second screenshot can only be enabled by View -> Play Queue, but that can't be done in the GNOME layout.
The settings wheel on the right in the second screenshot, beside the volume icon, might be a MATE / GNOME Classic specific extension, but would (in GNOME 3) have contained a lot of the items in the top menus. It's missing in 3.30.1 / 3.4.2 too though. (It's in Totem and Gedit with GNOME 3.30)
I looked in
.config/rhythmbox (doesn't exist) for the configuration file to disable the "Minimal" layout (as that could be the cause) but I can't find the file Rhythmbox stores its user preferences in.
Is it possible to have a top menu and plugins in Rhythmbox 3.4.2 / GNOME 3.30?