The value specified for generated column [whatever] is not allowed

I have a table - Lead.Metadata - with a column named meta that's of the JSON type and a generated column:

cycle VARCHAR(255) AS (JSON_UNQUOTE(meta->"$.cycle"))

The problem is that when I do UPDATE Lead.Metadata SET cycle = 'MONTHLY' I get this error:

Error Code: 3105. The value specified for generated column 'cycle' in table 'Metadata' is not allowed.

I don't understand. cycle is a VARCHAR(255) and 'MONTHLY' should fit in that.

Any ideas?

(also, fwiw, the codebase is a legacy codebase; the database structure is suboptimal but fixing that is a larger project)

Your column cycle is generated column, can not be updated directly.
Values of a generated column are computed from an expression included in the column definition.
Shortly: generated column is an expression.

Update main column, then this column recalculated automatically.

select id,cast(meta as json)meta,cycle 
from Metadata;

UPDATE Metadata 
  SET meta=json_set(meta,'$.cycle','MONTHLY')

select id,cast(meta as json)meta,cycle 
from Metadata;

fiddle