You’re running into a limitation of Lexicon: it can only merge properties (i.e., property ), not merge or transform property values within a property. [Merging data]
From the docs:
- Lexicon “merge properties” lets you take, for example, utm_campaign and utm_id as two separate properties and merge them into a single property name. It “does not affect the raw data” – it just makes Mixpanel treat them as one property in the UI. [Merging data]
- There is no documented feature in Lexicon to:
- Rewrite values inside a property, or
- Combine values from one property into another (e.g., move old utm_campaign values into utm_id), or
- Replace a placeholder value with (not set) / null.
So:
1. Why you can’t do what you want in Lexicon
The knowledge sources do not mention any way to merge or remap values of utm_campaign into utm_id via Lexicon; only property-level merges are supported. [Merging data]
2. What you do in Mixpanel UI (workarounds)
While not a true “merge of values,” you can:
- Use Custom Properties to compute a new property that prefers utm_id but falls back to utm_campaign (or vice versa) and then use that in reports. [Custom properties]
- Use this custom property everywhere in analysis instead of the raw UTM fields.
The docs don’t provide an explicit formula example for this exact UTM case, but they do state that custom properties can combine existing properties into a new one and be used like normal properties. [Custom properties]
3. About the placeholder vs “(not set)”
Mixpanel treats missing/undefined UTM properties as “(not set)” in reports; if you sent a literal placeholder string instead, that’s just another value and won’t be auto-mapped to “(not set)”. [Web attribution; Falsy values]
The knowledge sources do not describe any built-in way to bulk-convert a placeholder value into “(not set)” or null inside Mixpanel.
Given the docs, there is no supported way in Lexicon to merge or rewrite the UTM values themselves; the recommended in-product option is to define a custom property that logically combines utm_campaign and utm_id and then standardize on that for analysis.
