The reason why there are positive figures under sequestered carbon for concrete is because the concrete industry burn a lot of waste tyres to generate energy for cement production. The burning of the tyres causes a release of biogenic carbon. This is an example where a regulator (in this case Greater London Area) has tried to reinvent parts of EN15978 without fully understanding the methodology behind the standard. So, no, not all sequestered/biogenic carbon impacts will be a negative figure by default because there are processes that releases biogenic carbon.
We recommend that you send those results to GLA as is so that they may begin to realise their attempted simplification of complex supply chains is flawed.