Upgrade from 11.x to 12.1.4: client does not use updated sylink.xml

I've 2 SEPM: one version 11.x, another (new) version 12.1.4

I've upgraded some clients (remote deployment) and the clients are correctly updated but they do not use/receive the new sylink.xml

Result? New client (12.1.4) points to old SEPM.
If I remote deploy the communication package, the client moves to the new SEPM.

How is that possible and I could I avoid this behaviour?

Thanks!