Is it possible to retrieve flash-based encrypted disks content (SSD, cellphones, USB sticks, ...) after password wipe/replacement?

It is indeed possible that the block containing the key cannot be properly erased. That's why LUKS uses AFsplit to store the key on disk.

The idea is essentially to expand a key that would normally be stored on a single disk block, into a format that requires many more, ensuring that the loss of a single of those blocks makes the key lost. So when erasing the key, LUKS only needs to be able to overwrite one of those blocks.

The number of blocks is configurable so as long as it is large enough, being unable to erase all of them is unlikely.

You can read more about AFsplit in the LUKS on disk format specification, and see a sample python implementation (linked from a crypto.stackexchange question).