The Wikipedia entry for ISO 8601 (not the normative source, I know) states:

"A decimal fraction may be added to the lowest order time element present, in any of these representations. A decimal mark, either a comma or a dot (following ISO 80000-1 according to ISO 8601:1-2019,[25] which does not stipulate a preference except within International Standards, but with a preference for a comma according to ISO 8601:2004)[26] is used as a separator between the time element and its fraction. […]. There is no limit on the number of decimal places for the decimal fraction. However, the number of decimal places needs to be agreed to by the communicating parties. For example, in Microsoft SQL Server, the precision of a decimal fraction is 3, i.e., "yyyy-mm-ddThh:mm:ss[.mmm]".[27]"

The UA spec. is silent on the precision of a decimal fraction for the JSON encoding and this has resulted in an IOP issue that we need to resolve.