The spec defines a time here: http://hl7.org/fhir/R4/datatypes.html#time with the requirement: "Seconds must be provided due to schema type constraints but may be zero-filled and may be ignored at receiver discretion".

But the example for time (http://hl7.org/fhir/R4/datatypes-examples.html#primitives) is:
<time value="14:35"/>
which doesn't have seconds.