Updated 27 November 2020 (fixed by Adobe)
The release of Illustrator 2021 (v25.0.0) brought to light a new bug native in Illustrator for macOS that is affecting users of Astute Graphics plugins.
When working in Illustrator, it is possible that text entry boxes (such as numeric dimensions) can stop working for no apparent reason. We have found that they return to normal when the document is closed and opened, or Illustrator is restarted, and we haven't found the issue to risk any loss of work.
How is it provoked?
We have found a clearly repeatable sequence:
Open the Control panel (not shown by default in 2021, but accessible by clicking on Window > Control).
Switch to the Artboard Tool and numerically adjust the size of the artboard using the Control panel.
What can be done about it [updated]?
Adobe have released an update via the usual Creative Cloud app route for Illustrator 2021 (v25.0.1). This update fixes the issue.