That is curious... Are you using a derivative of the sample configuration and if yes, have you deviated in any way from the sample WITHINDLG route?
Still, I haven't tested specifically for this with 5.0 branch, it is possible I missed it...
However the reINVITE I tested was only in one direction. In this case, the 200 OK relayed by kamailio is not the same direction as the 200 OK corrupted by topos for the OPTIONs I reported originally. So my test maybe didn't replicate yours exactly (barring any config file differences).