The problem most probably has to do with the material that this black seal is made of (Delrin??/POM??). It is much harder and less flexible than PTFE / Teflon. When the reg has compressed the black seal for a long time the reg+valve seal will be "dented" by the pressure of the reg bolts.
The original Walther seals for the LG300 Dominator are white PTFE.
The newer Walther LG400 has a black Delrin/POM valve seat but has an o-ring on top to seal the reg. Best of both worlds. A hard valve seal and a soft reg seal.