Here is a weird one, easy to reproduce. 
In a blank default scene:

1) Create a primitive sphere
2) In VISUAL pane, turn on HDR
3) Change Glare from Spectral Cross to Disabled



4) Turn on Global Illumination (GI)



It only shows in viewport as pixelated when both (a) GI and (b) Viewport GI are on. 
But, try rendering -  it renders with pixelation if either (a) GI, (b) Viewport GI, or (c) both are 'ON', and can only render without pixelation if both are 'OFF'. 

Adjusting lighting settings or IBL image seem to have no effect on the phenomenon.  Only way to avoid is to turn off all GI settings.

Right...That can't be right, right?  I mean, am I right?

I can't confirm this. Bloom looks good on my PC, even with GI eneabled.

I am able to reproduce it, with an interesting caveat.

If I add a primitive, it is pixelated like that. If I add a more complex prop, it is smooth like it should be. The primitive also becomes smooth if something more complex is added to the scene. It does not seem to be material related, as I added a blank cat to my scene. Simple shapes with materials still cause the pixelating. It also happens with the different glare settings, just harder to see unless crank-up the bloom really high.

You should post your findings in the Feedback Tracker.

Wow, nice one!  Very interesting find by you.  I confirm your finding that by adding a higher res object (ie a CC avatar), it 'fixes' the pixelation. 
When I uncheck the avatar from the scene, in scene manager, the primitive's bloom goes back to being pixelated.
Do you think this might be because iClone may be outputting the bloom effect at a minimum-required resolution, depending on what object is in the scene? 
Thanks much!