way does MgCL2 (aq) + NaHCO3 gives no precipitate and way MgCL2 (aq) + NaHCO3 + Heat does gives a precipitate?

when i have a saturatet solution of MgCl2 and add NaHCO3 no precipitate forms but when the same solution is heated until boiling, there is a precipitate forming, whan cooling SOME "not all" of the precipitate dissolves agean. ph remains +- 7