STIntersects=true; STIntersection=null ??

Not really sure whether this is a 'bug' or me not just 'getting' the single-hemisphere restriction, but sometimes when STIntersects returns true, STIntersection returns null.

I first noticed it when 'drawing' LINESTRINGs across the 'international date line' (longitude=180) which is where the default map edges are -- the "line" from Sydney to Los Angeles necessarily crosses this boundary.

DECLARE @idl geography
DECLARE @idlSouth geography
DECLARE @SYDtoEquator geography
DECLARE @SYDtoLAX geography

SET @idl = geography::STGeomFromText('LINESTRING(85 180, -85 180)',4326);
SET @idlSouth = geography::STGeomFromText('LINESTRING(0 180, -85 180)',4326);
SET @SYDtoEquator = geography::STGeomFromText('LINESTRING(-33.9 151.12, 0 -118.4)', 4326);
SET @SYDtoLAX = geography::STGeomFromText('LINESTRING(-33.9 151.12, 33.8 -118.4)', 4326);

SELECT @SYDtoEquator.STIntersects(@idl) as [Intersects]
     , @SYDtoEquator.STIntersection(@idl) as [Intersection]
     , @SYDtoEquator, 'Sydney to Equator' as [Desc]
     , 'why is Intersection null?' as [Question]
SELECT @SYDtoLAX.STIntersects(@idl) as [Intersects]
     , @SYDtoLAX.STIntersection(@idl) as [Intersection]
     , @SYDtoLAX , 'Sydney to LAX' as [Desc]
/* shortening the 'idl' line 'fixes' it */
SELECT @SYDtoEquator.STIntersects(@idlSouth) as [Intersects]
     , @SYDtoEquator.STIntersection(@idlSouth ) as [Intersection]
     , @SYDtoEquator, 'Sydney to Equator' as [Desc]
SELECT @SYDtoLAX.STIntersects(@idlSouth) as [Intersects]
     , @SYDtoLAX.STIntersection(@idlSouth) as [Intersection]
     , @SYDtoLAX, 'Sydney to LAX' as [Desc]

This is the result "set"


And this is how the lines "should look"


As the line endpoint changes, so does the STIntersection result (although STIntersects returns true every time)

@SYDtoEquator = geography::STGeomFromText
    ('LINESTRING(-33.9 151.12, 10 -118.4)', 4326); -- NULL
@SYDtoEquator = geography::STGeomFromText
    ('LINESTRING(-33.9 151.12, 20 -118.4)', 4326); -- NULL
@SYDtoEquator = geography::STGeomFromText
    ('LINESTRING(-33.9 151.12, 30 -118.4)', 4326); -- GOOD
@SYDtoEquator = geography::STGeomFromText
    ('LINESTRING(-33.9 151.12, 40 -118.4)', 4326); -- GOOD
@SYDtoEquator = geography::STGeomFromText
    ('LINESTRING(-33.9 151.12, 50 -118.4)', 4326); -- NULL

So... any thoughts why some of these 'succeed' while others fail?