Probabely the reason could be hydrolysis reaction of Zinc chloride to form white precipitate of Zinc hydroxide - Zn(OH)2
Zn2+ + 2OH- -> Zn(OH)2(s)
When hydrated zinc chloride is heated, one obtains a residue of Zn(OH)Cl e.g.
ZnCl2·2H2O → ZnCl(OH) + HCl + H2O
It may also form Zinc fluoride -ZnF2 from the reaction of Zinc chloride and Sodium fluoride,
ZnCl2 + 2NaF = ZnF2 + 2NaCl
Zinc fluoride is practically insoluble in water and may form white precipitate.
As all above reactions are pH dependent, please adjust pH of mouth wash product appropriately to avoid precipitation.
However, please carry out compatibility studies of all ingredients used in the mouthwash formulation. Suggested to perform this by eliminating one ingredient in each experiment to find out exactly which ingredient is responsible to form white precipitate. You should have performed the compatibility studies at the time of product development in R & D.