Invoice Number Sequence and Invoices through the API

Until recently our API add-on (that we maintain) has been able to send invoices to MYOB and to also accept invoices that were created in MYOB. To avoid number clashes we prefixed the add-on invoices with 'HS' then used 6 digits (in a sequence). It seems that MYOB is now using that sequence to create internally generated invoices - which is exactly what we were trying to avoid. Our add-on generates many more invoices than are created internally to MYOB.

Question 1 is whether this is indeed the case: that MYOB is considering the add-on invoice numbers in the sequence generation.

Question 2 is how to address the underlying issue that the add-on needs its own number range to avoid clashes. I can think of some options but perhaps you can offer some alternatives in the first instance.