I have the same problem as yours, maybe try sim_residuals <- simulateResiduals(model_fitted,plot = T,n=>1) , try changing the n(number of simulations) as mentioned here (https://cran.r-project.org/web/packages/DHARMa/vignettes/DHARMa.html#general-remarks-on-interperting-residual-patterns-and-tests), additionally try adding dispersion model as well after checking plotResiduals(sim_residuals, data$pred1). Hope it helps.