When I start my laptop and do not login right away (about 1..5 minutes delay) the computer will go to Standby. Maybe this is related with https://bugs.kde.org/show_bug.cgi?id=354250
If you really think that this is a bug you need to report it to sddm: https://github.com/sddm/sddm/issues Personally I think this is expected behavior and not a bug. Anyway marking as resolved upstream as this belongs to sddm.
For me this is not expected behaviour because my machine runs on A/C power and there is no visible configuration that says that the computer should go to to standby. I reported this upstream: https://github.com/sddm/sddm/issues/562