When importing a single BPMN XML the reference is to the DMN decision table (or service) is not set.
Please use an App export / import for importing models and keeping (and updating when needed) references to other models.
It means that, without using an App, it is impossible to use and redeploy (manually) only a BPMN without having to set again each time the corresponding DMNs (and Forms?).
We always have to redeploy the full set of BPMN + DMN + Forms (in the App) just to keep the references in the BPMN?
For forms it’s a bit different. When importing a BPMN XML the static property formKey is set.
But for single BPML XML imports the behaviour for tables / services is as described. (One of the technical reasons is that you don’t have the context of all decision tables available when importing a single BPMN file. This is way you have to do something special in order to determine the name and id of the referenced decision table; which is needed in the modeler to make the link).
You could have a look at implementing something like the forms approach and contribute it to the project.