Hey again,
I just took matters into my own hands and found a great and simple fix. Looking at videos online I see that there's no spring in that latch, instead it seems to hold on by friction. I also took it off the grip to check if there's anything I'm missing (4 screws to take off cap, 1 screw to take off the latch). There's really no space/design for a spring mechanism (see photos). However I realized that underneath the latch there's a bigger recessed pocket. For a quick test I took some paper, folded it a few times and stuck that into that pocket so that there's friction with the axis of the latch. And it worked really well. I happen to own a 3D printer which can also print rubber (TPU) so I just quickly modelled a 0.8mm thick piece in the shape of that pocket, printed it and stuck it inside. It's working perfectly now and the latch stays in place where you leave it. You can probably use paper or cut out a thin piece of rubber to do the same (no need to remove the latch of course). I added a pattern file with measurements for that. Alternatively I will add the .stl file in case you want to print it yourself somehow.
Besten Gruß und viel Erfolg,
Andreas
Fuji_Battery_Grip_Latch.stl