Step to reproduce the problem:
1. Create a simple decision node with input: string output: string
2. Implement the decision node ad a table

it generates this DMN: SampleModel2.dmn
the validation engine says

"WARNING","[_95E9A8C2-34B5-453A-AAF2-9C8BDB092A03] DMN: Decision table with single output on node 'Decision-1' should not have an output type reference (DMN id: _95E9A8C2-34B5-453A-AAF2-9C8BDB092A03, The referenced element should not have a typeref set) ","SampleModel2.dmn","0","0"

In fact, the dmn contains the following line:

<dmn:output id="_95E9A8C2-34B5-453A-AAF2-9C8BDB092A03" typeRef="string"/>

removing the typeRef="string" the validation warning disappear.