Why would the lead lifecycle field change from Customer to MQL after sales has closed the lead and marked it as customer/sold? This is causing my nurture campaigns to resume for leads that are already sold.

Your screenshot shows Change Data Value, but because your question is about a lifecycle I would expect to see Marketo changing a revenue stage in a Revenue Cycle Model. Curious if you are changing data values to drive a Marketo RCM, or if you're simply tracking a lifecycle using only custom fields. Either way, if you don't want the data value to change to MQL after field named "Changed Lead Lifecycle - TR" has a value "Customer," you can use a Choice in the Change Data Value Flow step that prevents it from changing to MQL if the current value is "Customer."