Hello,
With 20.04.0, there is an issue when enable or disable an item in the startup applications. If we tick/untick an item fast and close, then no effect. If we tick/untick item then wait a delay (how?) and close, then the changes is done. Here a video recorded:
Thanks Philippe for the bug report! Very well done.
I can confirm this issue on 20.04 and also can confirm this doesn't happen on 18.04.
How to open a bug report and where?
On the Ubuntu Mate Launchpad Project, in the bugs session https://bugs.launchpad.net/ubuntu-mate
Thanks for opening an issue!