As someone who is fully drunk on Monzo kool-aid, well done to them on (a) identifying the problem and (b) immediately telling customers what to do. Imagine how long this would have been an issue if it had happened at Barclays or TSB.
I find it bizarre how people aren't giving Monzo more of a hard time over this. This is a security blunder of the highest order. Saying sorry in a cutesy email after the fact doesn't in any way make up for it.
No information was leaked... it's really kind of a non-issue. Monzo's approach is radical transparency. Any other bank probably would have never said anything.