I had a problem very similar to this 3 years ago.
My fuel/oil line got pinched in my jack plate. I was able to free it up and it looked fine once I freed it up. Then the problems began within 30 days after that until eventually a piston seized.
If that line gets pinched in any way it may not display visible damage on the outside. The damage maybe on the inside of the line.
The fuel/oil line is a relatively cheap fix.
Props go to Mercury, they covered the parts even though the warranty ran out. My fuel/oil line looked great on the outside, that’s why it wasn’t evident ay first.
Good Luck!