Stuck takedown catch

You are correct there is a "notch" that holds the takedown catch in place when pulling it down to field strip the P99
If this is on a new gun give it some time and after a few times the barrel lock and and take down catch will "pop up" by itself after you re-assemble the slide to the frame .........
If you take a look at the parts diagram.. the "notch" on the upper trigger holds the take down catch in place once it is pulled down....

its part #23 and #24 in this diagramm......you can't see the corresponding "notch" on part #24 on this diagram.... but look at your P99 and you can see what I am talking about
http://www.carlwalther.com/docs/ezt_2688450.pdf

hope that helps..... if this sould be on a used P99.... might want to put a drop of Hoppes # 9 on top of the barrel lock (part #28) and let it soak in.....

I wouldn't sweat it.....
the take down catch has a notch, so it will stay in place when removing the slide......you can let the take down catch come out of this notch before replacing the slide and see what it does then....
mine does not stay down... but mine is a 2004.... and you never know what kind of design features they may have changed over the years......
I guess what I am saying is... don't worry about it .......
or you can call Waltheramerica and see if they can help you out with the question if this is suppose to be that way