case insensitive collation of Greek's sigma

Hello,

during our tests of Postgres with ICU we found an issue with ILIKE of upper
and lowercase sigma (Σ). The letter has two lowercase variants σ and ς (at
the end of a word). I'm working with en_US and en-US-x-icu collations and
results are a bit unexpected - they are inverted:

postgres=# SELECT
postgres-# 'ΣΣ' ILIKE 'σσ' COLLATE "en_US",
postgres-# 'ΣΣ' ILIKE 'σς' COLLATE "en_US"
postgres-# ;
?column? | ?column?
----------+----------
t | f
(1 row)

postgres=# SELECT
postgres-# 'ΣΣ' ILIKE 'σσ' COLLATE "en-US-x-icu",
postgres-# 'ΣΣ' ILIKE 'σς' COLLATE "en-US-x-icu";
?column? | ?column?
----------+----------
f | t
(1 row)

I run those commands on the latest (14.1) official docker image.

Is it possible to unify the behaviour?And which one is correct from the
community point of view?

If I could start, I think both results are wrong as both should return
True. If I got it right, in the background there is a lower() function
running to compare strings, which is not enough for such cases (until the
left side isn't taken as a standalone word).

Thanks,

- jj