I am happy to say that i found a good workaround.
On other topics i found that it was impossible to make the string in “ISO format” without making use of this frankenstein: "current user date formatted as 2021"T"current user date formatted as 15:00:00"Z (the plugin was not accepting it either ).
The custom format does not allow me to type the T or Z.
But i found out that this was possible: yyyy-mm-dd"T"HH:MM:ss"Z"
By using the " i could place the T and the Z.
Now it works as if it was formatted as ISO for the pluging.
Thanks for the time to reply here.
I still think it is a bug and i will try to submit a bug report.