I don't know if this relates, but we have experienced this bug and it was finally attributed in our case SPECIFICALLY to our receptionist forwarding the call using the UCA Software Attendant Console. The CID is passed just fine if the receptionist uses the physical phone to make the transfer. CID will follows in that case, but not if she uses the software transfer function. This is a bug with the Mitel UCA and I finally gave up hassling our dealer to get Mitel to fix this after 2 major version updates and multiple tickets. When we purchased the system I went through so much assurance to our receptionist that things would be easier once she jumped into using the console I didn't have the heart to go back and tell her to go back and instead have endurred numerous complaints from sales about their inability to get CID to incoming calls from our office. They all show up as our office number because of this bug. Something for you to look at and may explain why it works at some and not others depending on if they are using the software console.
btw the "Propogate original caller ID on Transfer" flag list also has all the users in it, makes no difference.