This call not failing in this particular case was our choice that I'll try to explain.
The resource (in the REST terminology) used here is "parameters", not "lov". I mean I guess your URL looks like ".../raylight/v1/document/{docId}/parameters" (*)
The parameters you are using contain apparently a LOV which cannot be refreshed, but this is not blocking for the workflow. I mean you can refresh your document even without getting any LOV value. That's the same behavior as in BI LaunchPad.
Hence no LOV values returned inside the parameter, but no error from the REST call.
I don't know which version you are using but we added some months ago (I can retrieve the exact versions if you really need) the description of the error inside the lov element. For example:
<parameters>
<parameter optional="false" type="prompt">
<id>1</id>
<technicalName>pcHana Variable foodmart.variables/VAR_INTERVAL_MULTI_MANDATORY/FAMILY</technicalName>
<name>Family [foodmart.variables/VAR_INTERVAL_MULTI_MANDATORY/FAMILY]</name>
<answer constrained="true" type="Text">
<info cardinality="Interval" keepLastValues="true">
<lov hierarchical="false" refreshable="true" searchable="true" mandatorySearch="false">
<id>UNIVERSELOV_DS0.DO29</id>
<error>
<error_code>IES 10901</error_code>
<message>The following database error occurred: SAP DBTech JDBC: Cannot connect to jdbc:sap://XXXXXXXX:30015 [Cannot connect to host XXXXXXXX:30015 [Connection refused: connect], -813.].. For information about this error, please refer to SAP Knowledge Base Article 2054721 on the SAP Support Portal. (IES 10901)</message>
</error>
</lov>
</info>
</answer>
</parameter>
<parameter optional="false" type="prompt" dpId="DP0">
<id>0</id>
...
eric
(*) the logic would be the same for any other call ending with parameters or parameters/{parameterId}.