Traveling with I-551 stamp

Has anyone traveled internationally with an I-551 stamp or know of anyone who has recently traveled with it? One of my siblings has their I-485 approved in October 2022 and still waiting for a physical greencard (tried everything from calling USCIS, to House Rep etc.). They have a copy of their I-485 approval notice and an I-551 stamp on their passport that expires January 2024. Should they have any issues traveling abroad and returning to the US?

My mother traveled with the I-551 for the very first time back in 2019. She arrived in NYC and she did not have any issues at all.

thank you for your response! Did she have an MRIV btw? did it look like this?

https://www.uscis.gov/i-9-central/te...amps-and-mrivs

or is it something like this: https://www.immihelp.com/sample-temp...d-stamp-i-551/

my sibling has the latter only. I'm a little concerned the CBP may give them a hard time as not many may be familiar with just the I-551 stamp and no visa on passport.

She had like the latter one like your sibling. https://www.immihelp.com/sample-temp...d-stamp-i-551/
She did not have an MRIV. I was very concerned too as it was her very first time traveling after 21 years and her English is very limited. They did not give her any trouble about the stamp or asked her why she had to get it. I hope it is the same for your sibling.