Ok, so what looks buggy is actually (almost) a feature. Fontself stores all point positions at high precision but displays only integer values, which may lead to rounding issues.
For greater precision, there could be 2 possible solutions: either we’d expose the actual positions values (in your example it could be 0.267923 - but it’s not the most user friendly values to edit) or we could also provide a template that forces the position of all points and handles on a grid of 1000 x 1000 units (which may suffice for many typeface designs).