macOS Sierra always prompts for credentials for network drives

At home we have several Macs that mount, on login, some SMB network drives from various servers (OS X Server and Synology NAS). The credentials (username and password) for the network drives are stored in the user's keychain by way of the 'remember this password in my keychain' option the very first time the drives was ever mounted. As a result a command like 'mount' command successfully mounts the drives without the user needing to interact with an authentication dialogue. This has been working just fine for the longest time an all the way up through OS X 10.11.6.

In Sierra, any attempt to mount a network drive always pops up the authentication dialogue. The password may be prefilled (presumably from the keychain) but the user still has to respond to the dialogue by clicking OK. This is a huge problem for me as it breaks loads of automation scripts that I have that rely on using 'mount' (the problem is not specific to mount however; the same issue occurs if you use something like Finder's 'Connect to Server' command).

I've tried deleting all the 'network password' keychain entries so that they get re-creqted (they do) but the problem behaviour still occurs.

Anyone know if this is a bug or by design?