Missguided 50%

Hi guys, sorry if this is the wrong place. long time reader of site, But first time poster.

Today I was absolutely overjoyed when a 50% missguided code landed in my email Box.
Like most savvy shoppers, I tend to shop the sites until a good offer temps me into actually making a purchase.
So could you imagine my delight as I headed over to Missguided to nab the things I had wanted for a while.... only to find that the items I want are no longer there? They were there yesterday when i looked, even before that.
Now I understand fast fashion, I have worked in retail, but I am starting to find this happening A LOT on here. Most of the good stuff vanishes during great offers, most of what is on there is quite old stock.
And i kinda have a feeling that if i look when the offer is no longer on, i might find the things i was looking for.
The thing is its hard to prove unless you have evidence, and they can easily say 'oh its out of stock' (despite most out of stock items on there still showing, just showing no sizes available) Its i have noticed it on several occasions and right after the items are back.
Is this even legal? who is checking the online retailers.
Anyway, I left a sarcastic twitter comment, but i doubt anyone will respond.

Am i the only one who has noticed this? Are other online stores guilty of this trick?