Eggs – Mousseline sauce, use the white or the yolk of the egg

There are two likely causes of soufflé problems.

1) Egg foam. Be sure that you're whipping them to peaks (soft peaks, as I recall) when you first whip them. Then, when adding the 2 mixtures together in 3rds, be sure that you're properly folding in the whites; this is a very specific technique, and easy to over-do.

2) Oven heat. Use an oven thermometer to check your oven's temp - it's probably wrong. Put some extra mass in that oven (like a baking stone or brick) to reduce wild temperature swings during baking.

But, as in the linked questions in the comments, all soufflés fall to some degree.

Your recipe doesn't specify 55°C, and I'd be surprised if 5–8 minutes over barely simmering water only gets that hot. Indeed, checking for sources:

• McGee, in On Food and Cooking, says:

  When the temperature reaches 120°F/50°C, high enough to unfold some of the yolk proteins, the mix thickens, traps air more efficiently, and begins to expand. As the proteins continue to unfold and then bond to each other, the foam rises into fluffy mounds. The key to maximally light zabaglione is to stop the heating just when the foam teeters on the cusp between liquid and solid. Further cooking will produce a stiffer, denser, eventually tough sponge as the proteins over-coagulate

  so that leads to the conclusion that 55°C wasn't hot enough, as it hadn't yet reached "the cusp between liquid and solid".

• Hans-Dieter Belitz, Werner Grosch, and Peter Schieberle say in Food Chemistry:

  Egg yolk can be whipped into stable foam only at higher temperatures (optimum 72°C), the volume increasing about sixfold in the process. Above the critical temperature, the volume falls and the proteins coagulate. The protein coagulation is prevented by reducing the pH value, e.g., by the addition of acetic acid. This effect is used in the production of highly stable sauces

  so that'd imply you want 72°C, or maybe higher due to the acid (wine) present. As a side benefit, that'd also pasteurize the eggs.

In summary, I think you should have cooked it hotter.

I doubt it is going to firm much in the fridge, hopefully it is stable enough to not further liquify.