One standardized value does not match Mplus/lavaan

Hello fellow OpenMx-ers
In the interest of helping people transition to R, I have been converting the contents of an SEM seminar taught at our university from Mplus to OpenMx, also using lavaan for comparison. So I have a slightly complex path model with two exogenous variables, two mediating endogenous variables, and one outcome variable. The model produces the same fit indices and solutions (including estimates and SEs) in all three packages, and all but one standardized parameters are also identical. In particular, the modeled covariance between the residuals of the two endogenous (mediating) variables does NOT match, and I wonder if this is a bug somewhere or whether a different formula is used. Note that lavaan fully matches Mplus, it is only the standardized estimate in OpenMx that differs. In the demo data set used in the seminar the discrepancy is quite substantial (.68 vs. .30). I do not really know what this parameter should be divided by to be standardized, so I cannot check further myself. Code attached, using a dataset provided with lavaan, for easy checking; the problem is the same.