Strange error in Project Server 2010 formulas in custom field

Today, we've found strange error that disappeared after restarting IIS. We had a project level custom field (Flag) referencing task custom field as [Task Field Name].

For some very strange reason we were able to change value of Project Custom Field with formula in Project Pro (Server and PPro are with December CU). This was possible only in P Pro, not in PWA. After restarting IIS this behavior was gone;  Very odd.