They do!
Umgekehrt! Umarex is the parent company, Carl Walther the “subcompany”.
Check it again and tell us if you find a shield containing three crowns. Then explain to us why Walther would send a pistol all the way from Ulm up to Köln to be proofed - especially as the Proof-House Ulm (Beschussamt Ulm) proofs Walther firearms directly in the Walther factory, even though the Beschussamt is literally “just down the road” (3km) from the Walther works.
To answer the OP: Firearms manufactured in Ulm are proofed in Ulm and are stamped with the corresponding proof-mark (an antler). Firearms which are manufactured in Arnsberg are proofed by the Proof-House Cologne (Köln), either in Köln or in Arnsberg and are stamped with the above-mentioned shield containing three crowns.
There are also (historically) “genuine” Walthers which were manufactured in Zella-Mehlis (former DDR or East Germany), as well as Manurhin (France) - but you can find out about this by reading up on the Walther history (for example, on the carl-walther.de website).
Balor