Answer
Yes — there's a combined/aggregate character-limit that applies across all the fields being passed together in a single initial save (create) operation, separate from each individual field's own defined maximum length. If the total combined character count across every field included in that first save exceeds this aggregate limit, the save fails with this buffer-too-small error even though no single field is anywhere near its own limit. The confirmed workaround: on the initial save, only fill in enough fields to stay under the combined limit (leaving the rest blank), save the record, and then update the remaining blank fields individually afterward — since each subsequent single-field update operates within its own limit rather than the combined initial-save limit.