Ghost in the machine with that information alone unless the EMM caught and recorded something. Start with the easy and obvious get some fresh alcohol free premium fuel from a brand name station if you haven't been running XD100 repent and start, change the plugs, change the fuel filter. If it happens again move the throttle up and see if the rpm comes right back.
I had kind of the same issue. I have the 2013 Pontoon series 115 hp.
Since brand new with the stock 15.5 x 13 pitch prop I would hit 5100 rpm at WOT.
After a minute it would drop to 4500 rpms and stay there till i backed off the throttle and went back to WOT. Then it would repeat the process.
The dealer kept telling me it was the breakin ( I know these engines dont have a break in) Evinrude told me it was not propped properly.
AFter 3 props I finally found the Solas Rubex 15.5 x 11 pitch.
My rpm is now at 5700 rpms which is recommended and I don't lose RPMS at all when running WOT.
Every owner has to make their own choice the most convenient fuel or consistent fuel. I have personally seen water front fuel that was sometimes being sold with more than 1% water in it.The pier owner was adding additional cosolvent alcohols close to the max allowed by law in my state to keep the water in suspension/prevent phase separation instead of reworking his tank vents and fills so they wouldn't get water in them every time it rained real hard. When it didn't rain hard after a tanker drop his fuel didn't have crazy amounts of water and added alcohol in it. The direct injection process happens very fast under lots of pressure and relies upon careful placement/aiming of the injector, precise timing, and injector spray pattern to reduce/prevent fuel impinging on the piston or cylinder walls. BRP has full control of aiming and timing but an owner can negatively affect injector spray pattern through the selection and use of fuels that either/both chemically perform outside the range of normal motor fuel standards or those that over time create injector deposits thereby altering the pattern. Current generation direct injection motor vehicles (which are more ficht than Etec when it comes to technology maturity) are also now dealing with these issues. Engines aren't engines just because fuel X plus additive Y works in one does not mean it will in another. My club has two aircraft both came to us with motor gas(pump fuel) supplemental type certificates one is carbureted and has run great for years on brand name alcohol free 92 octane premium, the other is fuel injected and it runs so bad on the same 92 octane premium we run it on 100LL in spite of the much higher cost. Does that mean the carburetor is better than the injection? It surely does not because when running on the 100LL the injected motor does everything starts, idles, transitions, climbs and cruises better than the carbureted motor. In fact the fuel costs end up being very similar because the injected motor can meter fuel more precisely and run much leaner.
Your dealer should be the one to find out if it is a fuel delivery issue or a propping issue.
The latest motors seem to run much better when propped at or near to the top of the rpm range.
"There is never just one thing wrong with a boat";
-- Travis McGee, main character in a book series by John D. McDonald
The factory recommends that a properly trained technician service your Johnson or Evinrude outboard motor. Should you elect to perform repairs yourself, use caution, common sense, and observe safety procedures in the vicinity of flammable liquids, around moving parts, near high-temperature components, and working with electrical or ignition systems.
The information offered here is only general in nature and should not be construed as complete factory approved procedures, techniques, or specifications. Always use the proper service manual for your motor, up-to-date service literature, the correct tools, and have an understanding of how to proceed with troubleshooting and repair methods. If you are unsure or uncomfortable with a procedure, a situation, or a technique, enlist the services of a factory trained technician.