Any amount you were eligible to contribute under the last month rule does not become an excess contribution because you fail to satisfy the testing period. You simply owe a 10% recapture tax (on your 2018 tax return in this case) on the amount that you contributed that was in excess of the amount that you would have been eligible to contribute were it not for the last-month rule. Unlike IRA contributions, only actual excess contributions are permitted to be returned to you in this way. Because this amount is not an excess contribution, you are not permitted to have it distributed back to you as a return of excess contribution.
If you request a return of excess contribution from the HSA custodian, the custodian will likely take your (untrue) word for it that you were ineligible to make the contribution and make the distribution that you request. If the IRS later audits and discovers that you were HSA-eligible for December, the IRS will treat the amount improperly obtained as a return of contribution as a regular distribution instead, subject to tax and (if under age 65) 20% penalty, as well as hitting you with the 10% recapture tax.
