Therein lies the irony. The 5e Monster Manual changes, sometimes radically (since when are Minotaurs a "curse race"? That certainly counters lore in Dragonlance), lore of all sorts of creatures. And many people, apparently including you, think that's great. So, if it's great to rewrite the background of a large percentage of the monsters in the game, why is it bad to change the background of psionics?
They've been cursed for a long time. So, you're completely wrong. The default lore in my 2e MM includes such a curse for the Minotaur. So the 5e version didn't change anything in that regard. Dragonlance had its own MM with its own version of the Minotaur.
Campaign Settings can completely override the default lore for any monster or PC race in the game.
I haven't heard any complaints that the tie to the Far Realms is bad in and of itself. The idea has a pretty lengthy pedigree, even if it was never explicit. Yet, not adhering to 2e lore is now a bad thing?
Like I said, the irony is rather delicious.
As far as "is psionics affected by magic", well, I would hope they go the 3e route and sidebar the idea that "Psionics are different" to make every table happy. But, even if they don't, what's stopping you?
4e removed all the lore and that didn't make every table happy.
Many people have returned to D&D because much of the lore that D&D fans love has returned. This time around the designers made the effort to ensure that it returned. All I really want is for them to continue in that effort. I'm just not sure I'm seeing it with the Mystic (yet).