Why is cast array integer[] <--> text[] is not immutable.

Hi

I'm struggling to understand why this casts is not immutable:

integer[]::text[]
text[]::integer[]

The following are all immutable:

integer::text
text::integer
integer[]::float[]
integer::float

I hit on this while trying to make a gin index which cast from one to the other.

Why does the encapsulation of an array suddenly make this not immutable?

Thanks