Accidentals with arrow keys.

There is something I am wondering about. I think I might have asked this before, but I ran into it agin just now and really want to know if this is by design or that it is the result of some logic that I am missing.

If I am inputting notes in Bb major and have to write a Db I would like to write a D and then flat it by using the down arrow. Musescore instead turns the D into a C# which I do not want. I only want it to turn things into sharps when I use the up arrow.

I know I can flat and sharp things with alt-b and alt-h, but I am just curious as to why things work the way they do, especially since it seems illogical to me.