Hmmm, this is a stumper. Normally, mousseline does not melt at 80 degrees, or even at 90. It should be soft, airy and creamy, but still hold its shape. Mine always thins out and then thickens up (dramatically!) as I add the butter. Two things come to mind, the curd and the sugar syrup.
How liquid-y was the passion fruit curd at room temp? I see you did limit yourself to adding the amount specified, is that right? Sometimes if the fruit isn’t too acidic, the curd doesn’t set very well and is softer. I’ve heard you can add a little lemon juice to the puree to address this, but I haven’t tried it so I can’t vouch for its success.
As for the sugar syrup, re-check the weight of your sugar and the accuracy of your thermometer, a few degrees under temperature could be a problem. And you need to use all of it, don’t leave any clinging to the pan. After that, if you still want to tinker with it, you could try adding a little more syrup, but don’t increase the temperature.
Lastly, I love the texture a little alcohol creates with Mousseline, so I would recommend reducing but not eliminating the orange-vodka liquor. You could try using 1/3 the amount, or 1oz.