Temporary Total Loss of Power

I would suspect either a collapsed intake air hose (or filter - there was a rash of this type failure in Grumman Tigers and Cheetahs a while back - Lycoming engines), or something like others have mentioned above – perhaps an alternate air "flap" loose and blocking the intake?

If that's not it, I would check the exhaust carefully.  I have not experienced this personally, but have read several stories about exhaust baffles (inside the muffler) breaking off and temporarily blocking the exhaust, then eventually either blowing out of the exhaust stack or moving around inside the muffler housing so they were no longer blocking the exhaust pipe.  You (or your mechanic) will need a borescope tool to really "scope" out the muffler.  Sometimes you can hear stuff rattling around if you tap on the muffler, but not always...  If it's broken / loose baffles, it will happen again, and you might not be as lucky next time...

As for the "water in the air box" theory, I do not believe it would cause the symptoms you experienced.  Rough running, perhaps, but sudden smooth reduction in RPM?  No way!  Likely not electrical either, for the same reason – it kept running smoothly at reduced RPM.  That leaves fuel or air (intake or exhaust) as potential issues, but the fact that it kept running smoothly at reduced RPM makes "air" the more likely of the two...

Good hunting!

Since you had a low oil pressure indication then the abrupt loss of power I would highly suspect an electrical issue.  I would check connections for corrosion or looseness as well as check continuity while jiggling wires etc.  Electrical issues, especially momentary ones are hard to locate but next time power might not resume.

I would say, that an electrical issue is highly unlikely. The Rotax has two independent ignition circuits and that both of them fail and come back to life at the same time is almost unthinkable. So, it was most likely a problem in the fuel supply or in the air supply.

Are you using car gas with ethanol? Stop using that for a while and see if this happens.
Also make sure your carb jets are clean and your fuel filter is clean.

So fuel pump was turned on (fuel pressure increases), carb heat applied (rpm decreased as is typical when applying carb heat?) then you made a throttle change requesting more power, and shortly thereafter, your problem appeared. Have you confirmed fuel flow is correct with the boost pump on, throttle linkage is secure, and just how does your carb heat work?  Also, are your carb springs set to go to WOT if there’s a linkage failure?