Currently I have my service template configured with this property and the source as input data:
The property is correctly filled in with the value I specify manually when creating a service with this template:
However when I change the value directly via right-click on a service -> Properties -> custom and export the services as a .csv file, the values in the exported file have not changed. It only works one way, you can update the properties by changing the input data, but you cannot update the input data by changing a property the way I described.
Is it feasible somehow so that it works both ways?
Unfortunately, that scenario is not supported.
When applied, a service template uses a set of input data to generate a DataMiner Service. While it keeps track of the input data that was used, there's no link back from the locations where that input data was used (in your case: as a property value), and even if there was, there's no good generic way to deal with changes made after service creation.
Exporting a service template configuration exports the input data which was previously used to create the service. Importing such a file re-applies the service template using the provided input data.