Custom tool validation inside model builder

I have a custom tool built on a python script that has some code in initializeParameters in the ToolValidator. The code executes fine when I launch the tool, but if I add that custom tool to a model then the tool validation no longer seems to execute when I run the model. The code is population drop-down lists in the tool and I want those lists to be refreshable, but it looks like in order to make that happen I have to delete the tool from the model and re-add it; otherwise, the lists remain static.

The validation actually uses a python shelve to read in a list of values

in ToolValidation:

def initializeParameters(self):
import shelve
db = shelve.open(r"c:\mydata\lists")
list1 = db[0]
self.params[0].filter.list = list1
return

If I alter that validation code to try to change the list:

def initializeParameters(self):
self.params[0].filter.list = ["My", "New", "List"]
return

If I run the tool with this new validation code, it opens with the list updated correctly. But if I run the model that the tool was originally embedded in, it displays the old list--as if the validation code had not executed within the model. If I delete the tool from the model and re-add it, then it runs properly with the updated list. So it seems that ToolValidation does not run within a model for a custom script tool but instead the tools parameter values are static based on whatever values they had when the tool was originally added into the model.