The function SPChangePointType() in splinechar.c has a unsatisfying behaviour that I will try to describe in two GIFs:

1. If you change a single-linked corner point to a curve point, it will head to the next on-curve-node in the distance of the nice proportion = .39:
   
2. If you change a double-linked corner point, the behaviour is basically the same but the direction is defined by the vector from the next and the previous on-curve-node:
   

This problem occurs both with the 2020-11-07 release as well as the current main branch of FontForge.

I have tried to fix the problem by changing line 699 of splinechar.c to something like

if( pointtype==pt_curve ) { 
	if ( sp->pointtype==pt_corner ) { 
		if ( sp->prev!=NULL && sp->next!=NULL && prevlen!=0 && nextlen!=0 ) {
			unitnext.x = .5*(unitnext.x-unitprev.x); /* average direction */
			unitnext.y = .5*(unitnext.y-unitprev.y); 
			sp->nextcp.x = sp->me.x + unitnext.x*nextlen;
			sp->nextcp.y = sp->me.y + unitnext.y*nextlen;
			sp->prevcp.x = sp->me.x - unitnext.x*prevlen; /* prev is antidirectional to next */
			sp->prevcp.y = sp->me.y - unitnext.y*prevlen;
		}
		if ( ( sp->prev==NULL && sp->next!=NULL && nextlen!=0 ) || (sp->next==NULL && sp->prev!=NULL && prevlen!=0) ) 
			makedflt = false; /* just do nothing */
	} 
    else makedflt = true; /* original behaviour */
}	

to change this behaviour but it did not work as desired. Maybe we should consider changing the function SplineRefigureFixup() as well? If someone has a good hint, I will try to figure out a patch and make a pull request.