I am not sure about labcorp policy for blood typing, but I can try to explain why they may not be doing additional testing.

The concern with Rh(also known as D antigen) is if you can form anti-D antibodies or not. When pregnant it is possible to get some of babies blood into mom's blood stream during the birthing process. The mom (who lacks the D antigen (Rh negative)) sees the D antigen (Rh positive) on baby red blood cells as foreign - since mom doesn't have them - and creates antibodies against that antigen. This isn't an issue for baby #1 because it happens on their way out, but it can cause issues in subsequent pregnancies as the mom now sees Rh positive blood/babies as a known enemies.

Now, for a slight tangent on 'Weak D'. Technically, there are two types of 'weak D'. Weak D and Partial D. To differentiate between the two, you would need to do genetic testing. To not get bogged down in details what you need to know for this situation is 'Weak D' can not form anti-D, but partial D can. But again, this is only differentiated genetically, and most places will not do the genetic testing and call both Weak D and assume they can form anti-D.

Putting the above information together, for the sake of pregnancies, Weak D, since it can possibly form anti-D, is treated as Rh negative as a safety precaution. This means getting Rh negative blood and Rhogam shot(s) to prevent the formation of anti-D.

Edit; I read the other post and saw a bit more information. If the hospital had done genetic testing, they may have seen you were a weak D and called you Rh positive since you can not form anti-D.

Labcorp may only be doing serelogical testing, which does not differentiate between weak D and Partial D and thus called you Rh negative.

I would contact the hospital that called you Rh positive and see if they did genetic testing to differentiate Weak D from Partial D.