I take it by no changes, you haven't changed your lighting either (brightened/increased)?
Were the clowns or the angel juveniles when the anthias were added? Are you certain there hasn't been even the slightest bit of bullying from the angel or the clown fish as they've matured?
My first suspscion would be bullying.
When I first started to read your post, my initial thought was bullying from another anthias as they may have grown/matured enough to reach the point going from juveniles seek safety in the group to trying to gain dominance/hierarchy...
But with only two, after 4 months I would think one would've transition to male and the remaining female would accept being his 'harem' (of one) ... unless both were already juvenile males and just finally reached an age of seeking dominance over the other. I think that very unlikely though.
As for the clowns and angel, it may not take much bullying at all from one of them to intimidate a meek, non-dominant anthias.
For example, my exquisite wrasse merely chased a new lubbocki wrasse across the tank for a ~8 seconds, then used its 'chest' to push/pin it in a corner for 2-3 seconds. No nipping of any kind. Just a short 'pinning'. But that was all it took for the lubbocki to dart into a hiding hole somewhere in the rocks and never come out again, not even to feed.
Granted, it wasn't anthias, and was more timid than my dispars. But shows that with some fish it really doesn't take much to make them feel very threatened.
If it makes attempts to come out again, try to keep a very close eye on whether any of the other fish move even the slightest bit toward it ... or even just posture or focus their glare toward it ... right before it darts back into cover. May give you a clue as to whether or not it feels intimidated by one of the other fish.
