Tested in Linux Mint 19.1; Libreoffice 6.x
-- Open Impress
-- Insert two object the slide (object_1 and object_2)
-- Switch to: View - User interface - Tabbed
-- Click object_2 and chose: Draw - Back one and/or Forward one button
Result: The buttons isn't work.
The Tabbed menu Back one-Forward one and the Standard toolbar menu -- Drawing -- Arrange -- In Front of Object and Behind Object icons is the same.
Buttons isn't work
Buttons work well
User Profile Reset: Yes
Build ID: 65f81f6acdeb03b34033a1c6515618936d5f8eca
CPU threads: 4; OS: Linux 5.0; UI render: default; VCL: kde5;
TinderBox: Linux-rpm_deb-x86_64@86-TDF, Branch:master, Time: 2019-04-23_17:18:14
Locale: ru-RU (ru_RU.UTF-8); UI-Language: en-US
Thanks for the bug report I already found out that it didn't work. Patch is added to BUG 125285
*** This bug has been marked as a duplicate of bug 125285 ***