With out seeing it I'd say it sounds normal.

 

My z950m has to be at half throttle and not moving. If I am moving when I engage the blades, it’ll stall the machine. If I’m below half throttle, it’ll stall.

 

In my opinion, something is causing excessive friction on the mower deck. I had a Z930A for 10 years, and I always engaged the PTO at idle with absolutely no bogging. I currently have a Z994M, and I have mowed 2 seasons with no issues while engaging the PTO at idle.

 

I'd bet you didn't have a <25HP engine.

 

The Z930A had a 29HP Kawasaki, and the 994 has a 25HP engine.

 

994 has a 25HP engine.

Diesel?
My point is IMO the 25HP gas engine in the A915 is a tad small for a 54" deck, not to mention a 60" deck.

 

Yeah every gas engine machine I've ever seen has said right in the manual to engage the PTO at full throttle. Diesels have the torque to spin up a load at idle and idle speed is preferred to save wear on the PTO clutch, but an equivalent horsepower gas engine just can't manage it at idle so you use full throttle instead.

 

The OP said it developed a strange problem, so apparently something new happened.
I have two Cub Cadet Zero turns, both around 25 hp (one 24. Other 26 I'm thinking).
I've had them for years, I can engage PTO at idle if engine warmed up and no load (on driveway, both are gas). Typically I engage them half throttle.
So bogging isn't normal. Off of course I'd see how hard it is turning blades. Something is causing a load, binding, etc.

Sent from my SM-S205DL using Tapatalk

 

My z950m (27 hp) with 60”, the manual states to start PTO at mid-throttle. Ive got 14 hours and if I start the PTO while idle if I’m also moving, it’ll stall

 

I have to be above 1/2 throttle, actually about 3/4 throttle to engage the deck (60") on my 915E for the first time (cold). Once it has warmed up I can engage it at 1/2 throttle. I think the hydro's suck some HP when the oil is not up to operating temp.

I usually keep my other hand on the throttle and as the PTO engages I open up the throttle.

I gotta agree with @Gizmo2 , the 25hp doesn't have much of anything below full throttle.