Master Page Maps not being applied (Structured FM 2022)

We use Master Page Maps extensively in our documents. All of them are applied to body pages based on an Element appearing on the page.

We have a Master Page called Iconography

On our StructMasterPageMaps table, we have  rule to apply this Master Page whenever the ConvensionsHeading (Top-level) Element appears on the page:

On the Body Page, we have added this Element:

However, when we use the Apply Master Pages command, the Iconography Master Page isn't applied.

To ensure there aren't any typos, the Element Name was copied and pasted from the EDD into the MPM table. Likewise, the Master Page Name was copied and pasted from the Master Page Name window.

All our other Master Page Mapping rules (15 of them) are being applied correctly, except this one. The Console window also doesn't show any errors.

Any suggestions?