Marketo SSFS does not update lead datetime fields

Hi!
I'm implementing an SSFS that can set or change a field of a lead in the callback. The documentation I use: https://github.com/adobe/Marketo-SSFS-Service-Provider-Interface

Wait Until (API Name: waitUntil) - custom datetime lead field

This is the data I callback to Marketo. The format is described in the documentation.

The field mapping is set up according to the documentation in my serviceDefiniton and looks correct in the Marketo interface.

But the "Wait Until" lead field remains empty.

It's the same with the other datetime fields. There is no problem with the string fields.
What can it be related to?