Hello, Felix,
Most likely this indicates that the monitor you are outputting to is only capable of 1080i59.94, 60, 50.
Both UltraStudio Mini Monitor and DeckLink Mini Monitor are capable of:
SD Format Support
525i29.97 NTSC, 625i25 PAL
HD Format Support
720p50, 720p59.94, 720p60
1080p23.98, 1080p24, 1080p25, 1080p29.97, 1080p30
1080PsF23.98, 1080PsF24, 1080PsF25, 1080PsF29.97, 1080PsF30
1080i50, 1080i59.94, 1080i60
With the new DeckLink Mini Monitor 4K also adding support for
1080p50, 1080p59.94, 1080p60
as well as UHD and 4K
Ultra HD Format Support
2160p23.98, 2160p24, 2160p25, 2160p29.97, 2160p30
4K DCI Format Support
4K DCI 23.98p, 4K DCI 24p, 4K DCI 25p
Dmitry Kitsov,
Blackmagic Design technical support agent