Hello @Diki,
In general, we recommend that folks do not cross-post questions, as it's mostly the same people on the Qiime2 GitHub and the Qiime2 forums and we will see it either way.
Did you know that the Illumina platform has a barcode-hopping problem (that Illumina wrote a paper about 
)?
Those taxa in your NTC could be the most common taxa in your real samples, that were misassigned to your NTC through barcode hopping / crosstalk. This is part of the reason that removing all taxa found in your NTC from your full study is a bad idea!