 Solution

Passivation by the oxide film is a likely cause. As poorly conducting CeO${}_2$ covers palladium and thickens, electron transport through the deposit becomes difficult and the interfacial resistance rises. Transport of ions/defects through the film can also become limiting.

Thus abundant cerium in bulk solution does not guarantee continued interfacial deposition. The film can block the electronic or ionic path required to grow more oxide.

This is an oxide-film transport limitation during cerium dioxide electrodeposition.