OK, I found the problem, I need to use absolute paths to the bitmaps, here, like this:
{
"vstgui-ui-description": {
"version": "1",
"bitmaps": {
"handle_bitmap": {
"path": "handle-bitmap.png"
},
"background_empty": {
"path": "background_empty.png"
},
"knobLogic": {
"path": "D:/PROGRAMACION/VS2022/Basic_Gain/Basic_Gain/resource/knob.png"
}
},