It seems that this HSA custodian only permits an HSA contribution to be designated for the prior year if it is made by check and is accompanied by a deposit form explicitly indicating that the contribution is for the prior year. An HSA custodian is not permitted to re-designate the year for which the contribution is made after the regular filing deadline of the tax return for the prior year unless you had properly provided explicit instruction that the contribution was for the prior year and they failed to follow that instruction.
If the custodian were to change re-designate the contribution as being for the prior year they would have to issue a corrected 2017 Form 5498-SA to show that contribution in box 3. They are only going to do that if they were the ones who made the error.
