I'm inclined to say you're mis-using the wallwart, or it's defective. All such devices (in fact, any device that provides power) is designed to provide a regulated voltage within a range, based on an expected load or range of loads. A meter doesn't provide sufficient load.

Likely your 9V wallwart is just fine.

However, what would really be nice is if you told us a model number, or provided a link to this 'meter' you're referring to from HF, so we could look at it, and obtain specifications. It could be that you're completely misusing it.... just a thought.

Its the cheepo meter they give away there are several part numbers.
I'm sure I'm misusing it. It's designed to use a 9 v battery.
Going to try this guy

 

I also hooked it to two different bench power supply's, same results with both supply's.

So it didn't work properly with isolated bench supplies either?

 

So it didn't work properly with isolated bench supplies either?

No it did not. The reading is very erratic using the isolated supplies.
Using the one I built using Richard's Idea the reading was off about 0.4 volts but not erratic.

Here is a link to my solution:
https://forum.allaboutcircuits.com/...anel-meter-no-transformer.118110/#post-929539

I'm thinking the DC/DC Isolation chip above mite work (Post#21) The meter will be using the same 12v source
and not getting noise from the power lines.
Plus the 12v source will be isolated from the meter.
Using Richards idea it looks like the DC also needs to be isolated from the meter.
I tested the meter with a Component Tracer and the both test probes are common to the battery terminals +&-.
It's only $4 for the chip worth a try I need at least 5+ meters powered this way.