What is the name of the effect or concept behind getting along better with people after time apart?
In general, I would have said the opposite: there's a robust literature on familiarity driving liking, even with non-social stimuli like shapes:


  https://en.wikipedia.org/wiki/Mere-exposure_effect


This is also true with people. There may be some kind of an effect whereby people who are familiar and then separated bicker less after the break than before, but I haven't seen this studied. I'd suggest you look at terms like conflict in close relationships for psychological research, e.g.:


  Simpson, J. A., Rholes, W. S., &amp; Phillips, D. (1996). Conflict in close relationships: An attachment perspective. Journal of Personality and Social Psychology, 71(5), 899-914.
  http://dx.doi.org/10.1037/0022-3514.71.5.899