During the creation of a jack audio driver, it is checked whether the sample-rate of the settings object matches jack's rate. If not, it was adjusted previously via fluid_synth_set_sample_rate(). Due to the deprecation of that function and removal of real-time capability of the synth.sample-rate setting, a regression was introduced in 5fbddcecc3 causing the synth's sample-rate to be not updated.
This workaround obtains the synth via the settings instance and for now calls the deprecated sample-rate set function.
Tom M