The Group element was added into Camunda Modeler (And Zeebe Modeler), but it looks like it was not added into the Model API?
The group element is required for parsing the collaborations:
<bpmn:collaboration id="Collaboration_1xisnek" isClosed="false">
<participant id="Participant_1e6coet" name="hahahah" processRef="Process_16i55l0" xmlns="http://www.omg.org/spec/BPMN/20100524/MODEL"/>
<participant id="Participant_09g5ptg" name="CAT" processRef="Process_1s0oy1z" xmlns="http://www.omg.org/spec/BPMN/20100524/MODEL"/>
<participant id="Participant_16w34k0" xmlns="http://www.omg.org/spec/BPMN/20100524/MODEL"/>
<messageFlow id="MessageFlow_1bvvrw7" sourceRef="Participant_1e6coet" targetRef="Participant_09g5ptg" xmlns="http://www.omg.org/spec/BPMN/20100524/MODEL"/>
---> <bpmn:group categoryValueRef="CategoryValue_1a52ttg" id="Group_070c8r0"/>
</bpmn:collaboration>
...
<bpmn:category id="Category_0u17ipj">
<categoryValue id="CategoryValue_1a52ttg" value="bb" xmlns="http://www.omg.org/spec/BPMN/20100524/MODEL"/>
</bpmn:category>
...
<bpmndi:BPMNShape bpmnElement="Group_070c8r0" id="Group_070c8r0_di">
<dc:Bounds height="130" width="290" x="270" y="745"/>
<bpmndi:BPMNLabel>
<dc:Bounds height="14" width="13" x="409" y="752"/>
</bpmndi:BPMNLabel>
</bpmndi:BPMNShape>