Bob Puffer determined the reason why exceedingly long (9000+ words, 64kb) entries in Journal and Online Text Assignments failed is because the data/text fields in the 'mdl_journal_entries' and ''mdl_assignment_submission' are set to 'text' (which has a 64kb limit) vs 'mediumtext' (which has a 16mb limit).
He's created a tracker report for this bug here:
Online Text Assignment Size Limit?
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