Recently upgraded to Monterey (from Mojave) on a machine where I have a simple macro that looks like this:
Opening the alias file from Finder has the desired behavior. If the share is already mounted, it opens it in a new window in Finder, if it isn't mounted, it mounts the share with the shared password in the keychain, without prompting.
Any ideas of a simple workaround for this, or why this might have changed?
Weirdly this works perfectly. 
Have you tried an explicit "with Finder" in the "open..." action, rather than relying on the default app?
Yeah, same behavior with specifying "Finder" in the open action. The alias opens the network share if it's already mounted, but it won't mount it like it used to in Monterey.
Before going any further -- is there a particular reason why you are using an alias for this? For example, to open the admin share on the local server dev-mini1 over SMB using the password mypswd for the account me:
Or if you are running static IPs on your network you can use those instead of machine names, and you can also miss out the password if you want more security:
...and you can, of course, use an external IP address or an FQDN for your off-site address.
No? This actually works fine. Aliases into the trash. Thanks as always.