Changed one domain to Proton but it affected other domains.

I have multiple domains registered at Square. For years I have used gmail. My main email is name@first.tld. I never used name@second.tld, it was just set to forward to name@first.tld.

I decided to start using Proton Mail for name@second.tld. I deleted the old custom MX records there and set them up according to Proton's instructions. This did involve setting the nameservers to Squarespace defaults. So email is now working fine at Proton for name@second.tld.

I didn't touch a single record of name@first.tld, but now my email there is going to the same inbox at Proton Mail. I really want to stay at gmail for name@first.tld, because I have been using it forever and I like having everything in one place and do not feel like migrating.

Any help is appreciated!