I don't have a Yamaha but I know my outboard didn't produce much of a telltale stream running on earmuffs. It doesn't spit water when flushing through the hose attachment (Evinrude says not to run the motor in this setup, just hose).
Suggest you put it in a lake somewhere and start it, the water pickups are fully immersed rather than depending on questionable seals on earmuffs. Need not even launch it if it's not a busy time at rhe ramp. Just keep it on the trailer and start her up. Whether it pees or not, let it warm up, observe temp closely. You should "see" thermostats open and temp stabilise.
1. Pee stream good, warms up and stable at operating temp. Go home knowing not to expect it to pee when flushing on hose attachment only.
2. No pee stream, but motor warms up and holds at op temp. Blockage somewhere in telltale hose or fitting.
3. No pee stream, engine heading toward overheat. Observe water pressure - is it within factory spec? If yes, then check function of thermostat/s. If no/low water pressure, you have a water pump problem or a blockage in the delivery passages.
Yes the motor is under warranty, but no sense in having them replace a perfectly good impeller at your cost. Seeing that pee stream is important when the boat is in the water, but the other methods of feeding it water are imperfect.
I tried a set of earmuffs that supply water to both ears, that made my outboard pee like a champ. But for flushing and testing purposes, the single sided ones work fine. Enough water gets to the pump for it to cool the engine.