The output of the actuator block in a MNB-Vx sometimes shows 'NaN' instead of a valid position. The only way to clear it is to drive it fully open or fully closed. The issue is intermittent and it's not specific to the same installed controllers. It appears on different controllers at different times.
The actuator can be initialized to avoid displaying 'NaN'. The setting in the VAV actuator block is called InitMode. Here's a description of this setting from the WorkPlace Tech Engineering Guide Supplement (F-27356):
If InitMode = 0, and the actuator doesn't drive full open or full closed for a long period of time, the NaN value will continue to be displayed.
Ensure InitMode is set to either 1 or 2 so that it synchronizes after a power cycle or database download.