It’s not a bug because the tempo marking is created from scratch (and therefore no properties are set).
Change the tempo in the properties panel at “Tempo bpm”, then everything stays as you want it
I’m sorry, that doesn’t really make sense. I created a tempo marking, changed its size, then changed the tempo, and its size reverted. The size property should be independent from the tempo property. Who would create a tempo marking, purposefully resize it, then change their mind on a tempo (a musical decision), and expect Dorico to prefer the old size rather than the new (a cosmetic decision)?
Are you aware you are in the german speaking Dorico Forum?
You didn’t change the Tempo property, you double clicked the tempo indication and entered a new one. At this point, Dorico considers this a new indication, deletes the old and creates a new one. If you just want to change the metronome number, do so via the properties panel.
Haha, no I didn’t notice I was in the German forum! That explains it. Sorry about that. OK, thanks for the explanation. (Still not satisfied that it works this way, but…)