Probably some weird carrier RCS registration issue. My wife uses a s10e on EE (UK) and I use a Pixel 4 XL on Vodafone (UK) and I get RCS messages between us. Only use it for the delivery and read notifications but it works.

I do remember having loads of problems with RCS registration failing on the Samsung side, or just disabling itself.

Edit: Specifically I think it won't register whilst on WiFi and requires the correct mobile APN. I used to switch APNs to get a public IP address/ unfiltered internet