While working on a 1.13 to 2.5 migration, we had a few typos in attributes names. To debug those, we ended up adding breakpoints to vendor/ezsystems/ezplatform-page-builder/src/bundle/ui-dev/src/modules/page-builder/page.builder.js:291, where the block's attributes are mapped to their config by name.

It should be possible to provide better reporting of those errors, ideally as early as possible.

With a dozen of custom blocks or more and a manual migration process, it is very likely that such an error will occur. I haven't checked, but I expect that it may have caused the content of that attribute to get lost during migration as well.