Not sure if this is deliberate but I farmed a stack of 5 R avoidance cores only to realise they cannot be fused. I am not sure if this is a bug or intended although I do not see a rationale for it being designed such that it cannot be fused. I believe purchasing a rank up catalyst is most likely not to work either. I think this is similar to Jaybezz's post when his CC and CC resist cores could not be ranked up using a rank up catalyst. I have no intention to buy any rank up catalyst until I know for a certain that it will even work on my avoidance core.

yeah finally got enough control cores and nada.. can't fuse either.

It's probably because those cores are bugged leftovers in the drop tables from when all the cores were separate waaaaay back when alert was on PTS and they hadn't yet switched over to the new combined three stat cores that apply stats based on slot.

"Severity/avoidance/minimap radius" is the one missing core type out of the new ones, based off of what can be base stats on shell gear.