Error spotted and corrected, but why did it happen?

Fortunately I use some friends as proof-readers, because, as we know, spotting our own mistakes is more difficult than spotting those of others! However, an error was spotted yesterday in a project which annoyed me, because I can’t see why it happened, and it throws into doubt my trust that Dorico will do things correctly.

In the attached project, look at the full score view on page 3, bars 31-34. A simple 3/2 passage with side drum notes on the second beat each time. Now look at the ‘Percussion Score’ view: the first beat rest is missing in each of those bars, making the music look as if it is 4/4, although there is no time signature change. Looking at this passage in Engrave mode with note spacing enabled, there appear to be no alterations from the default layout in either view. So why are the beats missing in the Percussion score?

I corrected the problem by using ‘Reset appearance’ (although the missing rests appeared hidden behind the grace notes, and note spacing had to be modified), but it worries me that objects as fundamentally important as rests can disappear in a part for no apparent reason.
Cutdown Bax Symphony No.6.dorico (944.6 KB)

I still haven’t completely got my head around implicit and explicit rests, but when I turned on View > Note And Rest Colors > Implicit Rests, the half-note rests at the start of bars 31, 32, 33 and 34, all showed up as black with the rests after the notes showing as grey. Selecting those four half-note rests and pressing Delete changed them from black to grey and they then appeared in the percussion score.

Another interesting observation which might or might not be relevant if this is unintended behaviour and the development team are looking at why:
In the Percussion score, in each of those four bars, if I select the whole bar and open the Properties panel, Custom scale and Color are both enabled. Disabling Custom scale and Color causes the rests to show. It turns out that (for whatever reason) the rest had been scaled to 1%. I discovered this by selecting the first grace note in the group of three then pressing the left arrow key - this selected the half-note rest. Custom scale showed 1%. Also, either Color or Opacity had been set so that the note, even when scaled up to 100%, was still not showing. Disabling Color fixed that.