What mechanism leads to these spectacular iridescent patterns in clouds labeled as a "Fire Rainbow"?
this looks like mother of pearl clouds https://en.wikipedia.org/wiki/Polar_stratospheric_cloud
they are made of ice crystals very high in the atmosphere the only clouds higher in the atmosphere are noctilucent clouds.if you google mother of pearl clouds there is a lot of pictures and information.
as a result of global warming the stratosphere is getting colder this makes
mother of pearl clouds or rainbow clouds visible from larger areas of our planet then before(more info is needed to dokument this statement of mine).

An other alternative is irradecent clouds,They have many of the same properties as mother of pearl clouds https://en.wikipedia.org/wiki/Cloud_iridescence 

The last alternative is a pollen rainbow but they do often form in the direction of the sun or moon(it is a little hard to see if the sun is behind the clouds in your pictures)The colors of a pollen rainbow is often more vibrant than other types of optical phnomena https://soranews24.com/2019/03/09/high-pollen-levels-in-eastern-japan-create-pretty-rainbow-suns/