To add my piece to this discussion. My 1715 had/has a similar problem of the sprag clutch releasing randomly. I discovered two things .... when first purchased from the dealer it would only rarely disengage, as the tractor warmed up it would work all day and not disengage. A couple of years ago I changed the fluid and went with TS premium fluid .... the problem got notably worse and required the tractor to be warmed up even more. I use the PTO almost exclusively for a back hoe pump ... so I'd get it engaged and let it sit there and run the pump at idle for five minutes or so and usually fine after that. Strangely enough I found that the PTO engages immediately when the tractor is in reverse and you can usually shift to a forward gear and it stays engaged. In all instances with the tractor warmed up it engages and operates properly.
I've worked with industrial sprag clutches and motorcycles which use often use sprag clutches to engage the starter ... they are very sensitive to oil viscosity. If you are familiar with the way they work, they depend on the rollers making a very small movement to climb a ramp and expand against the outer shell of the clutch. Thick oil will prevent them from climbing the ramp "wedging" against the outer shell, that's why they work so well dry. I suspect that slipperier modern oils, with their better viscosity control are hindering the engagement until they are thinned out by increased temps.
Of course this does not address BROKEN sprag clutches where the outer shell has been damaged or the roller cage damaged due to poor design or excessive forces.
I've totally eliminated the bad behavior by altering how I operate the tractor and getting the gearbox warmed up good before using the PTO ... climbing a hill a couple of times will usually do it. Next fluid change I will try NH spec fluid, which I suspect will be of lower viscosity and I'm betting it will require less warm up.
PS. My unit was a dealership rental unit with about 900 hours when I bought it, so I'm sure it saw it's share of abuse .... I noticed clear evidence of the entire rear end being disassembled prior to my ownership ... I would almost bet that was to replace the problematic original ORC unit. Mine does not disengage under heavy load, it is more likely to disengage when load is disrupted, say pulling on a root and the bucket pops out of the ground ... which causes a momentary over run, the sprags release the outer shell of the OTC and the thick oil prevents them from easily moving back out again. Again, this behavior disappears when the gearbox is fully up to temp.