Two hypotheses exist. Either I am going totally nuts, or this is some sort of odd defect. I use tuplets spanning barlines heavily throughout my pieces for long notes. [I wish Dorico would provide an option to allow this without having to faff around with the tuplet hack, but that is not the issue.]
Using a dotted crotchet, if I add anything after it the quaver rest in the next bar collapses and the spacing is wrong. Try it!
Minimal working example attached. Dorico 5.0.20.2042. Windows 11 Insider Preview.
Click the tuplet number in the bass stave and set to āspans barlineā. The bars compress and do not leave the space that should be in the next bar (a quaverās worth).
This is the expected behaviour, for what itās worth. When thereās nothing āsolidā for Dorico to space, Dorico wonāt leave space. (This is the same reason, for example, that if you use the Ends voice property to prevent bar rests from appearing in later bars, those bars will end up very narrow.)
Youāll need some monkey business to get this effect if you have no other music at the start of the second bar. A hidden tuplet with a hidden quaver in another voice is possibly the best way to approach it.