I have encountered this several times already after different Cumulative Updates/Service Packs…
We set fixed maximum size for SQL Server databases and update the max size when required. But after installing Service Pack and Cumulative Updates the maximum size is reset to 2,097,152 MB for the MSDB log file:
I am going to submit this issue to the Microsoft Connect (hopefully it still works and active) or check if somebody else had similar issues. As this happens only with one system database and only with one file I would suspect that this is a bug.
This is just something to watch out and remember to review after SQL Server patching if you use fixed maximum files sizes.