As an "employee" who also has a late year heavy income, I have been randomly reviewed for the late year heavy withholding in the last 30 years. Never had a penalty, as it seems there is nothing improper (albeit unusual) for an "employee" to make most or all of their withholding payments with their last paychecks of the year. One instance was when the IRS did away with the W4 review process. They sent me a lock in letter, which was revoked after a phone call and record review - that I had not had any under withholding issues in the past). My income continues to flow in in a similar manner, and I still have no withholding in Q1-3, with no issues from the collection folks.
Anecdotally (I cannot remember the exact details), I know of at least one business owner who is in a similar situation, and while there have been inquiries, any penalty has been abated each time, since the quarterly deposits matched the quarterly income. In one case, the business owner "held" substantial checks from their clients until the new year, and in audit, as a cash based business, the check hold was not an issue to the particular auditor.