Devs found out more about this. RTKLIB marks positions as float on backwards pass if the fix differs significantly from the one on the forward pass. In this particular log file the fix could have been false and it was propagated by the fix-and-hold strategy through the solution. Going backwards RTKLIB detected that and flagged all solutions as false. When choosing “continuos” AR, which is less prone to holding erroneous solutions I was able to get a proper solution with combined filter.