Honda GXV690 loosened bolts between oil pan and crankcase? is that normal?

I have a Huster Z ZTR mower that I bought second hand three or four years ago. the former owner had swapped it's original Honda Engine for a GXV690 and it has been fine for the time I've had it. This season seems to be the one where everything wants to go wrong at the same time.  Had multiple relays from the Hustler side of the mower go out, but have that sorted and got it fixed.  Did an tune up of Oil, filter, and spark plugs on the engine side because it had been seeming a bit underpowered, figuring that couldn't hurt.  I'd noticed an oil leak started last year, but hadn't paid it much mind.  Once I got the mower running, I ran it for a bit and lost power to the right hand side drive wheel.  I was able to limp it back to the barn, did a bit of investigating and found oil leaking onto the drive belt causing slipping.  the oil leak was much worse, so I thought maybe I'd done something wrong with the oil change, but it checked out.  so I cleaned it up to see if I could find where the oil was coming from.
 
After a bit of looking over it, I found some oil near the front of the engine, on a bolt that was loose that connects the oil pan(which is mounted to the mower frame) to the crankcase.  further looking shows a few other ones being loose and I think one may be missing. so I've ordered some bolts in to ensure I have it bolted together strongly so I can put it back in service.
 
Has anybody ever seen a Honda GXV motor that loosened up the oil pan to crank case bolts?  It doesn't seem like something I should locktite, but not something I want to have to continue to chase for the rest of the mower's life. 
 
Anybody seen a motor do this before?