Hi N-drju,
Your maintenance includes all 4.5.xx builds, even if they are released after your maintenance expired. Our current policy is to try to keep the maintenance cutoff date the same for all builds within a minor version group (e.g. 4.5 is one group and 4.4 is another), although there may be exceptions to this. For Terragen 4.5.xx builds the cutoff date is July 1st, 2020. Frontier builds of 4.5 were available in July.
Recently we started adding this information to the public change logs here:
https://planetside.co.uk/change-logs/ . For example the 4.5.56 change log has a line that says "Included in Maintenance up to July 1, 2020." This information is also included on the download page which is accessed from your purchase history when you log into your account.
It might not be stated on the download page when you Check for Updates within the application, but if the update checker is working correctly it should only take you to versions that are included in your maintenance.