Just by nature of how they work - line filters will always have leakage current to ground.
Inside the filter is an array of inductors and capacitors. There are AC 'across-the-line' type capacitors tied from hot to neutral, from hot to ground and from neutral to ground. As long as there is AC power applied to the filter - there will always be leakage current. This includes when the power switch is turned off when the filter is often prior to the power switch. There is still power present within the AC filter in the diagram shown in post #38 hence there will be leakage current even with the switch turned off.

Leakage current is a actually a measure of filter performance. Contrary to common sense, the higher the leakage current, the better filtering. Yes, this seems counter intuitive. But if you think about it, high level analysis says better caps will filter better and conduct higher current during the filtering.
The line-to-ground capacitors provide a path for AC current to flow to the chassis and to ground. As long as the equipment has a good ground, this current will flow into the ground circuit and present no hazard. However, when the ground connection is faulty, the earth connection may be established by the human body. There is a maximum limit to leakage current for obvious safety reasons. US, Canadian and European safety puts the maximum leakage current limit at 3.5mA. Most filters used in pinball/arcade equipment is similar to the Corcom B series such as 5VB1. These have maximum leakage current of 0.4mA (120VAC power) or 0.7mA (240VAC power). Enough to feel it but not to kill or harm you. It takes about 10mA at 120VAC to physically affect your muscles and just over double that to affect your heart.

Sometimes these filters do go bad. Caps will either short out, open or become excessively leaky.
Open caps are not an issue but will result in less filtering.
Shorted caps will take out the entry fuse.
Excessively leaky caps can become a safety hazard.
Only sure way to test a line filter for excessive leakage current is to actually measure it between the filter's ground connection and the AC power cord's ground. Suggested method is to install a 1500 ohm resistor between the filter ground and actual ground. Measure the voltage drop across the resistor. Divide the voltage drop by 1500 to determine leakage current. Don't leave the resistor there - make sure to reground.

I didn't even think of such an easy method of isolating the ground -> an adapter plug.

The Corcom B series filter such as the 5VB1 has a maximum leakage current of 0.4mA.
You are seeing 0.57mA.
Not knowing exactly which filter your machine has inside, this very well could be the normal leakage current within the filter.
What brand and part number is on your filter?

5VK1 -- that one has a maximum leakage current of 0.5mA. Yours is a little high and is showing it's age. Debatable on replacing him but as long as the game is grounded, you should be fine. Some of the leakage current could be sloppy connections, though.

If you were to replace - a TDK makes a decent replacement. Pretty much the same footprint with a bit higher current rating:
https://www.greatplainselectronics.com/proddetail.asp?prod=ZHG2206-H

If it were me - I would just make sure all the solder joints were fine and stick with that one.