Should W-2 2018 section 12c W contain employer contributions from my payroll made in 2019 for 2018?

[Ashby]

That all depends on when the contributions were made and when the W-2 was processed.

However, you can add the late contributions:

there is a screen that asks where there any other contributions to the HSA? Drop down the box and you'll see where it lists three boxes, the last one being contributions not reported in Box 12 of your W-2.