BUG #17258: Unexpected results in CHAR(1) data type

The following bug has been logged on the website:

Bug reference:      17258
Logged by:          David Calascibetta
Email address:      david@calascibetta.com
PostgreSQL version: 12.8
Operating system:   AWS RDS
Description:

My understanding is that a CHAR(1) column can never be 0 (zero) length.
I expect the results of these two SELECT statements to produce the same
result:

select length(substr('   '::varchar,1,1)), ascii(substr('
'::varchar,1,1));
produces==> 1    32   (as expected)

select length(substr('   '::char,1,1)), ascii(substr('   '::char,1,1));
produces==> 0     0     (not as expected)