Phono cartridge left / right output voltage differ

That translates into (almost) exactly a 1.0 decibel imbalance. It'll be quite audible, and some (myself included) will find it extremely annoying. Others are oblivious to channel imbalance until it's more severe.

Unfortunately - until you get into the big money cartridges - many models are spec'd by the manufacturer at no better than a 1.5 dB channel differential. Some models are spec'd at 1.0dB, and once you get into the high end, 0.5dB is more common. So your cart is almost certainly considered normal - especially since it was stated right on the spec sheet!

You could try to mitigate this by playing off other imbalances that may exist downstream. E.g. if you have an imbalance due to asymmetric room acoustics (and you don't mind swapping L/R). Or, you could have a custom 1.0dB attenuator built and place it between the weaker channel's phono -> preamp link. Or you could return/sell that cart and keep looking.

It's irritating when this happens. We expect "new" to be perfect and free of problems. I bought a DL-103 from AudioCubes years ago and it happened to me, but I don't remember hearing a difference between channels.

Should you complain to the vendor? I don't have a strong opinion one way or the other, but I wish you luck.

The difference is ~.1 dB. No, I don't think you can hear that.

This difference is about a .5 db difference. That is well within most cartridge specifications. One can hear a .5 db difference as a subtle shift in the position of the center image in a set up where the center image is narrowly drawn, but, the shift will be hardly an issue (if it is an issue to you, then you MUST have a balance control, preferably one that can be operated by remote control.

The left is .16 mv and the right is .18 mv.

That is great for a Denon.