From Squarespace Help: "I tested this out and looked into what you described, and it does look like we have some reports of Text Block content not being indexed and available in the site search for some version 7.1 sites. I’ve made a record of this for our Engineering and Design teams to review. That being said, there are many moving parts and variables that we need to address before we roll out a solution. We thoroughly test all fixes before they’re released to ensure they don’t affect other Squarespace components or customers. I don't have an immediate workaround or solution to offer for this, though.As a result of these variables, we can’t guarantee how long it will take to fix issues like these. Also, because of the volume of requests we receive, we can’t always follow up personally on cases like these when the issue is resolved."
Because only "some version 7.1 sites" have this problem, I will have to redesign this site over on 7.0.
Well, at least I have time on my hands right now thanks to self-isolation. Neat.