I got nothing from the 50 rainbow star chest, I checked customisations and keys.

There was a glitch with the Chests properly displaying what you received so they changed it a few weeks back so that your reward goes into the Sticker Shop as a gift pack; so if you're waiting for something to show over the chest you'll just see a Sticker Pack briefly but nothing else.

You'll need to visit the Sticker Shop and you should see a notice in the lower left corner that you have Packs available to open; you should be able to access them by using your Triangle button on the PS4 controller.