The key advice now is to WAIT for guidance from IRS on this. Extend your filing deadline (from July 31 to October 15), if this is not done automatically by IRS announcement in June or July. With funding deadlines later than October 15, 2020, IRS has to give some relief in the deadline for filing 2019 Schedule SB to at least 1/1/2021 (if not the end of January 2021). It would be crazy for IRS to do nothing, forcing us to show "Unpaid Contributions" as of the filing deadline that are actually not due until later.
For any plans that do file a 2019 Schedule SB before the extended Funding Deadline, showing an Unpaid Contribution , I think they should be filing an AMENDED 2019 Schedule SB after the minimum funding has been completed before the extended 1/1/2021 deadline. The 2020 Schedule SB should not be showing any Unpaid (2019) Contribution if the 2019 contributions were timely deposited before 1/1/2021.
.... Jeff