Sounds like the client lost the state key, or some other client-side 
bug. My guess is that the client is sending all the client data back 
up to the server for some reason, thinking they are new or changed, 
but doesn't include all the fields that are on the server, so they are 
blanked out. (Normally missing fields are checked against a 
"SUPPORTED" array sent from the client that tells the server which 
fields can be ghosted etc... )

To be sure, I would need to see the synch log. Not necessarily the 
entire log, but would need to see the first SYNC request from the 
client to server that triggers the resync - as well as the first few 
response/request cycles since there may have been error codes sent etc..

Scenario:
- Contact existed on an Android 4.4.4 device
- Editing contact in Android 4.4.4 or in Turba updated both devices properly
- Device was then upgraded to Android 5.0, which uses a new Exchange 
framework integrated into the Gmail app (instead of a separate email 
app)

Bug:
- When updating contact information in Turba, a sync initiates with 
the Android 5.0 device.
- The activesync log created by Horde shows the entire address book resyncing
- All the contacts email and location information is blanked out (For 
instance all information in the Location and Communication tabs of 
Turba are now blank)
- Updating the email address in Android 5.0 Contact app properly 
synchronizes, and the email address is displayed in Turba
- Updating that same contact email address in Turba results in the 
email address being replaced by what was last entered on the Android 
5.0 device

All in all, I believe whatever is on the Android 5.0 device is 
overwriting whatever is already stored on the server. I'm not sure 
what to provide that would be helpful from logs, as my activesync logs 
are chock full of these full resync's (3GB+ for each user). Let me 
know what I can provide to clarify.