There is a misunderstanding about this due to errors in practice.  Strength training requires systolic and diastolic muscle movement, which has a regular rhythm of tightening and loosening.  When muscles are highly contracted, congestion will occur, leaving a lot of metabolites, and the length of the muscles will be shortened accordingly.  At this time, if contraction continues without stretching and relaxation exercises so that muscles can extend fully, then muscles will shorten in compensation, resulting in decreased flexibility.  Clearly, then, strength training is not the factor that impacts flexibility.