The check seems to be on the length of the whole string.
With a column of type StringList of size 5, '["AA"]' fails with 'Must be 5 characters or less'.
No network, nothing in console.
Definitely client-side. We currently don’t have any table cell editors for list type columns (I’m waiting for the design, which was working on at the end of last week).
We don't have a nice editor, yet, but we have fixed the client-side validation (in ).