Move (M) an object on a plate, the drop down provides the “world coordinate” box with X, Y and Z positions of the centroid of the object.
Any change applied in these boxes moves the object the same amount but in the opposite direction. For example, x = 50.50 and I edit to x = 50.00, it will then move to 51.00. However, if I edit to x = 50.99, it will then move to 50.01, then I edit to 50.02 and it will then arrive at 50.00!! Whoopie!! 
Please FIX!!! 
Are you on the latest version of Bambu Studio (1.9.4.60)? If not, you may want to update your software. I’m currently running the current version and I attempted to replicate your problem, but I was unable to. Using your example, I started at x=50 and attempted to change to x=51 and it moved to 51 as expected.
If you are on the latest version and you are still experiencing the issue then you’ll need to submit a big report. Not sure how much these kinds of bugs are looked at here, but I’d suggest submitting the bug report on the Bambu Studio GitHub which can be found at Issues · bambulab/BambuStudio · GitHub. Be as detailed as you can with your issue, including software version, OS version, and steps to recreate your problem.
I’m on version 1.9.5.51 and moves for X and Z seem to work, but Y is messed up. After a center X and Y are 90 (A1 Mini). If I change Y’s world coordinate to 78 as soon as I hit enter, the Y changes to 102 and the object is positioned at 102. If from a center, I enter 102, it changes to 78. However, if the object is not currently at 90, the change in Y is wierd. If I change 102 to 90, it ends up at 114. If I change 114 to 90, it changes to 138.
Update, I just did some more editing of the model I’m creating and now Y positioning seems to be working properly.
More wierdness. If I center the part containing these objects, it has X,Y of 98.5, 90, but if I center the objects in the part, they get 90,90. Yet, both the object and the part appear properly aligned at the center of the bed.