Majustis is an instantaneous curse breaker, it doesn't erase sympathetic links or provide ongoing protection. Otherwise Alchemist wouldn't have needed to follow up with Mind Blank.
Alchemist obviously realised there would be a blast radius given he immediately moved into a bomb shelter in the Dreaming after being in a completely different persona from his usual self.
Majustis erases any and all magic on a target, not just curses. Alchemist used it on John and chained it to everyone with sympathetic links to John of any kind. This included the alternate selves and twins, as well as any demon etc that had lain any claim to John's soul/body/mind, or anyone else who had active spells or effects on John. So Majustis affected John, all of John's alternates, John's alternate twins, and any other family John had within 2 degrees of separation. It erased any and all magical effects on any of those, as well as any of John's (alternate) family within a further 1 degree of separation (since 1 degree was used up by following the links to John's alternate family). Majustis also followed those sympathetic magical links (curses, magical contracts, etc) as they were erased and affected the ones who laid those on John. So any demon, devil, angel, fey, or other spellcaster that had any magical effect active on John got hit by Majustis, negating any and all magical effects on them. Then Majustis followed any such links on
those beings and erased any such effects affecting those. It stopped there, because those were the beings within 2 degrees of separation.
That was Majustis. It followed all sympathetic links and erased the ones it could as it went. It couldn't erase John's sympathetic links to his alternate selves and family, because they aren't magic, so they were all that existed after Majustis erased everything else, as though they had never been there, because that's what Majustis's effect is. It completely unravels and erases magic, and no form of detection in the multiverse could detect those effects after that, because they were erased completely and utterly.
Then Alchemist cast Mind Blank on all the sympathetic links that were left. Since Majustis completely destroyed all the magical sympathetic links, Mind Blank could not affect
any of the beings that weren't related to John via his family ties, both immediate and alternate. The demons, devils, fey, angels, and other spellcasters that had lain effects on John could no longer be affected, because those links no longer existed to be followed, so none of them got Mind Blanked.