Something is not quite right here, but the various theories about what might be wrong are not, I think, correct. From looking through the code I can see that Dorico does correctly retrieve the prevailing time signature. I think the confusion is caused by the fact that Dorico cannot straightforwardly “add bars”, because Dorico doesn’t think in bars: instead Dorico has to calculate how much rhythmic time would be occupied by the number of bars requested, and then by extending the flow by that amount of time, bars are naturally given rise to.