(SOLVED) Transmission slipping on reverse after more than an hour drive

[Yosia]

Hi, newb here. I hope someone will give me way out from my issues.

I drive BMW 2003 X5 E53 4.4i.
Recently, after I drive more than an hour, doesnt matter drive normal or hard or on start stopping traffic, my transmission slips while in R mode when I want to park my car after drive. Well this scares me because reverse gear is needed to park the car here almost all the time. On D mode, theres no slip at all but sometimes it downshifts roughly from 2nd to 1st after around an hour drive also. My transmission had been overhauled and had all the clutches, all the seals, and everything that came with the repair kit were replaced with the original BMW's repair kit a month ago, then the mechanic filled the ZF5 transmission oil from zero to almost 12 liters which means the fluid was flushed and new. All the job done by the BMW mechanics but he couldnt figure out whats the problem. I've read this forum talkin bout overheating transmission, well I'm not sure since no one slipping on the reverse gear after an hour drive.

I hope anyone could help me, thanks.