At the beginning of this TPR WG I raised the issue of preservation of DNSSEC status when a domain registration is transferred. There wasn't much interest, and this issue was generally treated as out of scope for this WG.
Google Domains' sale of its registrations to Squarespace apparently resulted in all of the registrations becoming unsigned. The numbers are big enough to have resulted in a drop in the overall count of signed domain names, and hence a topic of conversation.
I do not have further information. I expect it will be easy enough to talk to the relevant parties to find out more. That said, Google said to its customers: