yz 125 sudden loss of compression

Look for the rings to be stuck in the piston ring groves. If not that, check the ring end gap in the cylinder.
If those check out ok then you may have blown a crank seal.
What did the spark plug look like, color?
Any marks on the piston skirt?
Did the engine run ok when you first fired it up or was the rpm way high?
Were the carb jets clean?
Was the fuel mix fresh?
Was the new piston furnished to the shop doing the bore job at the time the bore was done or was it ordered after the bore job was done?

Paw Paw

If that is the case, you need to find out why it stuck as it will do it again if not corrected when you put in another new piston and ring.
It can happen due to over heating, poor fit, incorrect ring end gap, a lean condition or by not allowing the new top end to properly warm up before running the hell out of it. You see, engines that have a sleeve in them take more time to warm up and really don't transfer the heat as well as the OEM plated cylinder does and thus you have different expansion rate between the piston, rings and the sleeved cylinder. This difference will cause this type of issue.
I would suggest going back to an OEM plated cylinder.

Paw Paw

Did you have the cylinder bored and then bought a piston. Or did you buy the piston and have the cylinder bored to match?

Post up a few pictures of the piston.

Did the shop that bored the cylinder chamfer the ports after boring ? Sharp edges of the ports will take out a ring in about .0005 seconds of run time. Surprising how many shops do not complete a bore job with a good port chamfering.

Exhaust port bridge should also be relieved a bit on steel lined cylinders.

The piston should also have lube holes at the exhaust bridge.
A few photos of the bad piston would be nice.

Paw Paw

Show us a picture of both bad pistons.
Oil in the gas?
Oil holes in the piston?

Paw Paw

As project racer said, check to ensure the exhaust port bridge has been relieved a few thou. Approximately .005", I think.

Here's a picture for an idea.
And the Link---> http://www.pilotodyssey.com/ExhaustBridgeRelief.htm

What are the piston to bore clearances ? Wiseco's are OK pistons, but have seen them vary a bit in size on pistons marked as the same. Are there any scuff marks on the piston ? Did it pin the ring down in the groove again ?

If so, clearance sounds tight. Wiseco (forged) pistons need a little extra clearance as they expand a bit differently under heat.

Is the bore good and straight, minimal taper from top to bottom ?