After a call with Siddharth we solved the issue.
For future reference, if someone has similar issue – we solved it by projecting first to EPSG:4326 (also known as WGS 84) as intermidiary step before projecting to local Hong Kong specific coordinate system.
The bug was possibly caused by floating point arithmetic error in the reprojection maths (we are using third party .NET library for this, so it’s hard to debug, but this is my guess). In general, using EPSG:4326 as a ‘home’ projection before reprojecting to anything else seems to be a good practice in these cases.
