Hi Gspeak,
This is actually a bug on our end at the moment we are working to fix, and may take a little explanation here.
Basically what is happening, is that when you set up a Customer as a 'Guest', they are prompted to set up a Method ID (the same type of ID you yourself would use to log into your account) What differentiates a guest Method ID from a regular one, is that a guest typically doesnt have access to the Full account, just the guest page, which is accessed by a specific link.
So when you reset a password for a Guest (which is the same process as resetting a password for a regular account), the page redirect takes you back to the regular signin page, rather than the Guest Page redirect login.
in the meantime, if you were to share the link again, or revist the shared link, you should be able to log into the guest page without issue.
Thanks Gspeak.
-Ben