Custom field not updating in lead info

Greetings,

We are noticing that a specific salesforce custom field we created doesn't update with new information whenever a lead interacts with a form. We sent out a campaign last week with the hidden custom field populated with a descriptive campaign code. When we started getting leads coming through, we noticed that some had our new campaign code while other leads (who had already been in our system) retained old campaign codes, some from a year or two ago. I went into field mapping and it appears that this field is not being blocked from updates.

Has anyone else experienced this? Any suggestions would be appreciated!

Thanks!