contentModificationDate customisation in ui_custom.xml breaks sorting

catorarn

env: win2ksp4/TS65SP2 (multiple TS servers affected, DEV/UAT/PROD)

Uncommenting the below commented contentModificationDate item in ui_custom.xml customisation breaks the sorting in CC PRO.

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Our objective is to have Name, Modified By and Modified sortable, but it seems that only Name and Modified can be sortable. Does anyone have an example of a customisation of contentModificationDate in ui_custom.xml which maintains its sorting behaviour?