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SurveyJS
Open-Source JSON Form Builder to Create Dynamic Forms Right in Your App. With SurveyJS form UI libraries, you can build and style forms in a fully-integrated drag & drop form builder, render them in your JS app, and store form submission data in any backend, inc. PHP, ASP.NET Core, and Node.js.
Should have just gone with my gut before posting - it was one of my external repos. In my case it was the repo for https://github.com/shiftkey/desktop but for anyone else reading I would recommend temporarily disabling all non-Pop OS sources (whether you do that via the Pop Shop/Gnome Software or through the terminal with something like sudo mkdir -p /etc/apt/sources.list.d/temp/ && sudo mv /etc/apt/sources.list.d/*.list /etc/apt/sources.list.d/temp/) and then slowly re-enabling repos one by one and running updates until you find the problematic one.