Quote From Rory:
I am a little confused about the idea not mentioning it.
I would do so, but do you think it is a lie?
Plus, when they ask my supervisor for reference,
my supervisor might mention that I switched to this new study
although she has no clue about my bad performance.
Rory
Been here, chose badly with a second post-doc after finishing my PhD.
It wasn't possible in my case, but if your supervisor mentions it and you're asked, just say in interview you didn't see the point of it once finishing your PhD.
Most post-docs want a quiet period once they finish a PhD (i.e. get head together) and ploughing into a Masters is anything but a quiet, chilled period after finishing.