JACK cannot represent tempo ramps in any sensible way.
It sounds as if there may be a bug in setting the JACK tempo information, but even if this bug is fixed, you should not expect JACK to be able to keep multiple applications in sync if Ardour is ramping. JACK’s model of tempo just does not include ramping. If it works, it will only be because the changes are slow enough or small enough for the errors not to matter.
Ardour will not use tempo information from an external source. Again, there is no standard way to represent tempo ramps in a way that can be shared among applications.