I mean with race that it had a ultra bolt-in sprag, billet front clutch retainer, billet servos, 4.2 lever, kevlar bands, accumulator rod and a TF-2 like AUTOMATIC valve body.

Yeah, the sprag is probably fine, no gears in forward work at all!

Yeeeees John, now that you say it, I think it did behave like that! I hooked off the column shift linkage because I first thought there is something wrong, but indeed, to me it seemed that it was going forward when in neutral at the moment the reverse wasn't working!
I tried to get reverse out of the parking but it wouldn't, but in neutral it would go forward. I am not 100% sure, but when trying this gears that's what I felt but I thought first I am in the wrong gear, although I shifted those gears many many times because I was hoping that it would suddenly work! So I think 90% sure that it was going forward in neutral... what does that mean?

But yeah, how and why can that happen on a fresh rebuild transmission???
I used by the way fresh Midland Type F ATF fluid and the fluid level was at max.