I'm going to squeeze two questions into one post here (both about thunderbird).

1. What might be the cause of this display problem with Thunderbird?
It only happens with Thunderbird and only sometimes (i've yet to see the issue in any application other than Thunderbird - including Firefox - since installing Leap a month or two back).
This is in Leap 42.1 and has been present across all plasma updates (5.4.2, 5.4.3, 5.5, 5.5.1 & 5.5.2)

2. Thunderbird still mostly uses the Tango icon set (I think that's what it is).
Will we ever see it fully implement the Breeze icons? (it uses some of them already - see the junk, draft and bulk folders in the image above). The mix of icons just seems a bit messy with the way it currently is.