@boots that seems like it's just evaluating +{} in the latter case..?
@boots {} can be interpreted as a block, but it doesn't make sense for it to do that here
[]+{} is just [].toString()+({}).toString which is ''+'[object Object]'
@edef {a: +{}} returns NaN in the same conditions as {}+{} which is interesting
@edef (also, []+{} is "[object Object]", so)