The amount they contributed for 2018, whether in 2018 or by the Filing due date for the tax return for tax year 2018 (which means the 2018 contribution was made in 2019) is supposed to be on the 2018 tax filing. You will want to Amend theirs.

You only get a 5498 when there was activity. The 5498 in May 2019 will state which Year the contribution is for, so use that to confirm they made that 2019 contribution correctly in time and identified as for tax year 2018.

A contribution made in 2020, properly identified as for Tax Year 2019 and made timely to the requirements will trigger a new 5498, and it will show it is for tax year 2019, even though it is issued in 2020.

Which means, look at the paperwork.