I have been made aware of a breaking change in SharePoint on Microsoft 365 that affects Modern DFFS.
The new “Microsoft lists” layout that is introduced does not support SPFx Command Set Extensions (this is how the Modern DFFS is loaded). This layout was not supposed to be rolled out to lists that has command set extensions or field customizers applied, but this does not seem to have worked as intended.
According to this thread on GitHub the SPFx team is aware of the problem and is working on a fix right now: https://github.com/SharePoint/sp-dev-docs/issues/9514
Hopefully it gets resolved soon.
Updated 19:45 CET: The problem is fixed – thanks to the quick turnaround from the SPFx team! Just clear the cache in your browser (hit Ctrl + shift + r or clear it from the settings menu) and it should be back to working order.