If you chose the "Phone" account as the DEFAULT contact account in BirdieSync settings on your Android device, here is what could be the reason of this behavior:
- Contacts are synchronized a first time from Thunderbird to the Phone account on the Android device.
- Android silently moves these contacts after a few seconds to the Google account (BirdieSync has no control over it) or after a reboot (depending on the manufacturer).
- At next synchronization, BirdieSync cannot find any contacts in the Phone account, because they were transferred to the Google account. So the synchronization wants to delete them on the desktop.
To validate this hypothesis, you may have a look at the number of items present in the Phone account and in the Google account which are displayed in BirdieSync Settings in the "Contact" tab.
The solution is to avoid using the Phone account as the DEFAULT contact account and no longer synchronize this account. Use your Google account instead. Note that you can use this Google account to store your contacts and at the same time fully disable the synchronization of this account with the Google cloud. To do this, open Android menu "Settings/Accounts & Sync" and uncheck Google synchronization to disable it.