When a TM is selected to a project, it is set without write rights by default. On the other hand, when a new TM is created to the project, it has write rights by default.
Available workaround (if any):
N/A. Write checkbox needs to be clicked on manually every time.
There is an option to decide whether the selected TM will have write checkbox enabled by default or not.
The client who uses TMs always with write rights would like to avoid the repetitive click and potential danger of forgetting to do so.