Did you ever solve this problem? I am having the same issue after my domain was transferred today. I do not have a Google Workspace subscription, but the Squarespace domain console says that I do. My email still seems to be forwarded, but no forwarding rules are listed and I cannot add any forwarding rules in the Squarespace domain console.
When I look at the MX records, they list Google SMTP servers, but as I said I don't have Google Workspace. Before the transfer Google domains handled the forwarding. I had other domains that were transferred, but after the transfer their MX records pointed to Mailgun rather than Google, and those other domains have rules that I see in the Squarespace domain console.
Should I delete the MX records that point to Google, and will Squarespace then create a new set of Mailgun MX entries and let me create email forwarding rules in the Squarespace domain console?