ActiveSync devices changed server address

I have Exchange 2010 where all users have mailboxes
And have ActiveSync devices (mobile phones) connected
Users conncted to Exchange ActiveSync server via let's say exch2010.contoso.com

I've introduced Exchange 2016 server and configured Activesync Access via name exch2016.contoso.com

Now all mailboxes are still on Exchange 2010 but I found out that all ActiveSync devices changed server
from exch2010.contoso.com to exch2016.contoso.com

How that happened??

It is not desired for me right now
Because I still have some tests to run with Exchange2016 server
But now all users are connected thru Exchange 2016 server

Is it possible to switch it back automaticall to old exch2010.contoso.com server??

How that is happening ?? Autodiscover in not configured from the external
nor thru the root domain name (contoso.com) nor thru autodiscover.contoso.com