I enabled Power strip in AppMenu panel applet in 26.04. It does not seem to have effect. E.g. shutdown does not work.
With UB 26.04, you sometimes have to try twice for things like applets to work: if ‘Log out’ didn’t work the first time, after trying the other four options, it works too. 
This is linked to your mate sysmonitor issue - its a polkit dialog issue.
I also have this issue, with lock, logout and shutdown =/
I need folks to give this a test
sudo nano /usr/share/applications/org.buddiesofbudgie.BudgiePolkit.desktop
Change the line Exec= to read
Exec=sh -c "sleep 15 && /usr/libexec/budgie-desktop/budgie-polkit-dialog"
CTRL + X to exit and save. Logout and login.
This will delay budgie-polkit-dialog from starting for 15 seconds - hopefully allowing things to settle down.
After this the power strip seems to work.
If you can gradually reduce the sleep number i.e. 15 - 10 - 5 - 3 - 2 - 1 I would appreciate the feedback when it becomes unreliable and starts breaking again.
This will allow a proper pull-request upstream - and a future SRU into 26.04
The sleep thing does help maybe. I had this issue sometimes with the highest value of sleep.
This is with another notebook, though.