I submitted a bug report about this. Party phasing problem - #42 by Dominiquê-proudmoore
Blue acknowledged, but no update; probably because it may be intentional.
Someone on another thread said they suspect it has to do with wPvP exploits, like forming and reforming raids in different phases to try to take advantage of the wPvP events (i.e. Nazjatar). If that’s the case, it’s a pretty lousy way to fix that because it punishes people who actually want to quest with their family and friends easily.