BUG #17170: Invalid collation created with provider icu and initdb' locale C

The following bug has been logged on the website:

Bug reference:      17170
Logged by:          Andrew Bille
Email address:      andrewbille@gmail.com
PostgreSQL version: 14beta3
Operating system:   centos-7
Description:        

Hi!
When using postgres built "--with-icu" I get an invalid collation with the
following script:
initdb -D db --locale=C
createdb testdb
psql testdb -c "CREATE COLLATION test (provider=icu,
locale='en_US.utf8');"
psql testdb -c "SELECT 'a' < 'b' COLLATE test;"
ERROR:  collation "test" for encoding "SQL_ASCII" does not exist
LINE 1: SELECT 'a' < 'b' COLLATE test;
                         ^
or 
psql testdb -c "DROP COLLATION test;"
ERROR:  collation "test" for encoding "SQL_ASCII" does not exist

Reproduced on REL_10_STABLE..REL_14_STABLE.

Thanks!