Redactor fields not saving more than 252 characters?
The cause of this ended up being that the field_introduction column for the Redactor field in the craft_content table was set to varchar(255), which means it can only hold 255 characters.

No idea how it got that way. The Redactor field (under its advanced settings) has never had an option to be varchar(255), only text and mediumtext.

Regardless, going through all of the Redactor field settings and re-saving them correct adjusted the column data type back to text and resolved the issue.