I have tried to reproduce this issue and found that no problem for me here. I get that its working for me also I can select category successfully. May be you already know the process but I have decided to instruct the process once again.
First, create a category. Assign category 1,category 2 under parent Category and so on…
Second, Go to Form. Select Category Tab from Post Fields Section.
Third, Enter a Title for Field Label
Fourth, Select Dropdown, Multi Select,Checkbox,Ajax anyone and test each one individually and let me know which one is working for you then I can understand the problem completely.