I have not run into this, but I have a few thoughts. First, are you using SSMS 2012 or a newer version? I know with some things (SSIS comes to mind), if the SSMS version doesn't match the SQL version, you can have headaches with doing certain tasks.
Alternately, you say this is a SharePoint database. Any chance the SharePoint service has permissions that would allow it to change database settings like that? I could see a 3rd party tool having its own "best practice" setup and forcing certain database settings such as the log growth. OR it may be that you need to configure the database settings inside the tool (SharePoint). I know my company has a 3rd party tool that uses SQL Server as a back-end and it needs create database permissions so it can make the database with the settings they see as a best fit for the tool. Once the database is created, I can remove those permissions, but it creates it with permissions that I don't fully agree with, but changing some of them makes the tool give alerts to all users (and gives admins a "click here to fix this setting" button). I would not be too surprised if SharePoint is making those changes for you OR if you can (and need) to configure it from within SharePoint.
Just my 2 cents. I could be completely out to lunch here. Do you have any other DBA's that sit near you who may see you are changing it and they change it back just to mess with you?
Grant's suggestion of XE I think is going to be your best bet to track down what is causing it to change back. Alternately, may not hurt to check the logs. Could be there is some error with changing it but for whatever reason SSMS is not presenting it to you and it MAY be in a log.