This is a follow-up bug of bug 111306.

in https://cgit.freedesktop.org/libreoffice/core/commit/?id=64ff7453408e578b95073db1740bfef4f500a2e4, the master slide background was added to the sidebar, however, the changes done in the sidebar are not applied to the document and thus, the changes are lost after some other changes or RT, for instance:

1. Open Impress
2. View -> Slide Master
3. Select a background in the sidebar
4. Change the orientation to Portrait

-> The background setting will be undone

or 

1.Create slide(s) in impress.
2.Open master slide view and change background colour to black. It is observed that the background colour changes to black and fonts to white, as intended.
3.Close master slide view. It is observed that the new settings have been implemented in all slides.
4.Save the presentation in pptx format.
5.Close the window.
6.Open file again with libreoffice.

Due to the number of reports we got since 64ff7453408e578b95073db1740bfef4f500a2e4 I decided to hide the widget in the sidebar in https://cgit.freedesktop.org/libreoffice/core/commit/?id=c57f10b67fceef47d304d74dce93be7389a7720c as a temporal fix to avoid confusion, however, the root cause should be fix at some point.

Note that the master background can still be changed from the page properties dialog.

Some info:
The code related to the slide properties dialog is in https://opengrok.libreoffice.org/xref/core/sd/source/ui/func/fupage.cxx#389.
As observed, it uses mbMasterPage to differentiate between masterpages and not masterpages... we should do the same in https://opengrok.libreoffice.org/xref/core/sd/source/ui/sidebar/SlideBackground.cxx