BUG #17761: Questionable regular expression behavior

The following bug has been logged on the website:

Bug reference:      17761
Logged by:          Konstantin Geordzhev
Email address:      kosiodg@yahoo.com
PostgreSQL version: 11.10
Operating system:   tested online
Description:

Executing:
select regexp_matches('a 1x1250x2500',
'(a).*?([1-9]\d*)\s*x\s*([1-9]\d*)(?:\s*x\s*([1-9]\d*))?');
returns: {a,1,1,NULL}
while executing:
select regexp_matches('a 1x1250x2500',
'(a|b).*?([1-9]\d*)\s*x\s*([1-9]\d*)(?:\s*x\s*([1-9]\d*))?');
returns: {a,1,1250,2500}

Shouldn't both results be equal?

Tested online at:
https://extendsclass.com/postgresql-online.html
and on ubuntu version 9.5