You can specify a fix start slide under slide show -> slide show settings.
When the selected start slide is before the current slide the presentation is started at the current slide.
Expected behavior: the presentation is started at the fix start slide.
When the start slide is after the current slide the presentation starts as expected with the choosen start slide.
reproducible with LO 18.104.22.168 (Win7 Home, 64bit)
fixed with commit e8b924e440a8f82076389d2774d1c414c87924dc