Fluke 1587 strange readings

[350mdk]

Hi all sorry for the newbie question
I recently purchased 2 used units of the fluke 1587 without Bluetooth....the original
The one tester works great
The other is giving me a strange reading on insulation test
At 50v  with open 55mega ohm ...good
At 100v with open 110 mega ohms ..good
At 250v with a open 275 mega ohms...good
Now at 500v with a open should be 550 mega ohms but meter shows 70 and climbing up
Same at 1000v same happens
Any chance someone knows what's going on
All other functions work perfectly absolutely spot on
It's just those 2 r not
Thanking you in advance