I think it's a bug too.
1/16" is the smallest it will make a fillet even if you set the fillet for 1/64".
Now zoom in and resize the arc made with the fillet tool graphically. If you make it smaller than 1/16" it will just display the radius dimension as 0.
The radius dimension should be able to display a dimension as small as the dimension settings set in Project Preferences.
I can't imagine needing a fillet this small but the program should be consistent.
The importance of accuracy and percision with dimensions is not just for the sake of documentaion. It's also important for the sake of comparison even when precision is not important.
ArchiCAD 25 7000 USA - Windows 10 Pro 64x - Dell 7720 64 GB 2400MHz ECC - Xeon E3 1535M v6 4.20GHz - (2) 1TB M.2 PCIe Class 50 SSD's - 17.3" UHD IPS (3840x2160) - Nvidia Quadro P5000 16GB GDDR5 - Maxwell Studio/Render 5.2.1.49- Multilight 2 - Adobe Acrobat Pro - ArchiCAD 6 -25