Flaw regarding input of Pedal marking

It is not quite true that if you drag a pedal line to the score from the palette, you need to unselect it by clicking on an empty part of the score before you can re-select it and modify its placement using the controls in the inspector (although that will work). You can leave the line 'blue' and then double-click on it in order to put it into 'edit' mode. Once you've done that, the controls in the inspector will take effect.

There is a difference within MuseScore between a "line" and a "line segment": a "line" is the whole thing, a "line segment" is just the portion of that line on the current system, for lines that extend multiple systems. So a line can have multiple segments, although a line that is contained within a single system will have only a single segment. Anyhow, I say this because this is the source of a number of little glitches like this, where some operation requires the "line" to be selected but actually only a "line segment" is, or vice versa. My guess is that is what is going on here. We normally try to manage this well, but sometimes some case slips through the cracks. Feel free to file a formal bug report.