Marvel didn't know what was up with Oscar Isaac's Moon Knight accent either
Oscar Isaac was iffy on doing another big-budget extravaganza until Moon Knight's Steven Grant entered his life

When the Star Wars sequel trilogy ended, star Oscar Isaac wasn’t exactly jumping to get back in bed with Disney or tentpole movies in general. That is, of course, unless he “needed a second house or something.” So it was surprising that Isaac needed a second house or something and took the role of Marvel’s Moon Knight only a year later. There was, apparently, another stipulation: Moon Knight must have a confusing British accent.
As we all know, Moon Knight suffers from dissociative identity disorder, which he expresses via four distinct personalities. One of those personalities Isaac imagined as a British guy named Steven Grant, whose accent is “weird on purpose” due to his “believed heritage,” Isaac said earlier this year to defend his accent.
If that wasn’t already confusing enough, per Isaac’s more recent interview with The Radio Times, Grant “wasn’t necessarily written that way.” Isaac came up with that accent for the character. The actor’s muse cares not for canon.