Mixed methods studies seldom require a philosophical underpinning. Instead, if you have a research question that is indeed well-matched to a sequential explanatory design, that is more than good enough.

Thanks David L Morgan but I learnt because it's a mixed method design, I can consider pragmatism as the underpinning philosophy......

Certainly you cam but there is no real reason to.

I am conducting a mixed studies systematic review for which the data analysis approach is a qualitative results-based convergent design however the theoretical underpinning related to review objectives is critical realism. Critical realism embraces the notion of multiparadigmatic research, supporting the mixed studies decision.