You’re between a rock and a hard place unfortunately, as this post describes:
But let’s apply a little positivity for a change - there is a workaround
In the Monzo account that you want to adjust the ‘missing’ Summary dates, tap on search and search for ‘Summary’ or ‘Spending report’. Hopefully a ton of spending reports will be listed, tap on the first one and you’ll get into the old Summary to change the date.
