Early 2000s S250 Bobcat that I've had about 10 years. I normally use it for moving logs, but I got a set of fork tines for it last year and realized quickly while using it to pull an engine that the lift arms drift down fairly quickly. I'd guess 1-2 inches a minute, maybe a bit more. I have a regular forklift for most of the forklift duties, but it'll about get stuck on a damp fart, so the skid steer is handy for off road forklifting. I repacked the lift cylinders when I bought the machine, since has maybe 2000 hours on it. They don't leak externally, lifts loads fine. I'd rather not rebuild them in the hope that it'll fix this unless I know for sure. A new spool valve is around $3500.
As you have discovered, a skid steer does not have anti drift valves, like a forklift or crane. That drift rate sounds normal, and is likely in spec
Not sure if there is a spec, but it wasn't like this when I bought it 10 years ago. I'd be afraid to get out with the bucket in the air as it'd probably rip the door off as the arms drift down.
Use the forklift to hold arms up, cap off one cylinder at a time and see if it will hold. Then cap both cylinders. If you only have one cylinder leaking through capping both can give a false result, as the good one will hold it. If doesnt leak down with cylinders capped it is a valve issue, but could be emergency lowering or self level valve as well as the main spool valve.
my 1st guess (without pulling the cylinders apart) would be to make sure the pedals ( if it's a manual control machine) arent sticking/rusted 2nd would be the emergency boom down valve cartridge is leaking, or the stem is stuck