serverStatus.transactions.lastCommittedTransaction.writeConcern records the write concern of the last committed transaction. This subdocument does not have a fixed schema: it may be empty or not, and the w field may be either a string or a number. This causes frequent schema changes for FTDC, and that can severely limit FTDC retention.

A record of the last committed transaction has limited diagnostic value, so the best solution may be to omit the entire lastCommittedTransaction section from FTDC.