Captured WC51 with M2 upgrade lost its weapon and cannot attack.
When playing as OKW today I found this weird issue and I made a rough test.
As shown in the screenshot, if a WC51 with M2 MG upgrade done was captured, it will have no weapon, the gunner will remain in this crouch position forever, and the vehicle cannot attack. Also it will have a MG42 icon.
On the other hand, if a WC 51 without MG upgrade is captured and be upgraded by the new owner, it will receive its M2 and the gunner will shoot the MG normally.