Answer
Open the Custom Logical Unit itself, go to its Attributes tab, and remove the specific reference field(s) that were pointing at the now-removed source fields, then synchronize the Custom LU again — this redeploys/regenerates the Custom LU's own view without the stale references. After that, recreate the page layout, or update it directly from the Custom LU's own 'Custom Pages' tab, to reflect the corrected field set.