Not a problem in my world. Not sure I'd even really call that a girdling root. Regardless, someday way in the very distant future, the tree will get old and die. I doubt very much that root will have played any part. My advice would be not to try hacking/chipping/cutting or somehow trying to remove it. It's a big, established tree. Let it be. As they say in med school: first do no harm........

let me put it this way ... i hate poplars.. lol ... the problem you saw in 5 or 10 years.. on the poplar.. you WILL SEE on the maple.. in 30 to 50 years ... as to your age.. and if that will be an issue in 30 to 50 years.. i do not know ... of all the trees in this wide world.. i wonder.. why you are limiting yourself.. to ONLY these two options ???? what about oak??? where are you .... and are you open to other options???? another way to say this.. is that anything that claims to be FAST GROWING.. which is the ONLY salable point of a poplar ... is going to be fast to be a problem.. and fast to die ... as you have learned ... and one might suggest.. if this maple is marketed under the same idea.. of fast growing.. well.. my gut tells me.. its might be another problem tree .. but again.. its the time frame of how fast it will all happen ... ken ps: i hate maples too.. lol .. but that is because i can not garden under them ... and if you had poplar.. i will presume.. you werent gardening under them anyway .. so it doesnt matter ... but if you have any inclination ... to want to do such ... you need something beyond a maple .. pps: you will probably be killing root suckers from that poplar.. for upwards of 3 years.. even if you have the stump ground out ... they will need to be snipped and treated with roundup or stump killer.. until they stop coming back ... let us know when this happens ....

Compared to their larger growing cousins, Japanese maples and their other Asian relatives have very modest and completely non-aggressive root systems. But like any tree, as they mature there will be lots of roots present under the soil surface. And these roots tend to be extremely sensitive to any disturbance - injuring them in any manner can lead to some very serious problems. I typically suggest designing and then planting the surrounding bed at the same time as you plant the tree, thereby reducing the potential for damage later on. And if you can limit your plant selection to low shrubs, groundcovers or spreading perennials that do not require division - plants that require minimal attention or cultivation once established - your maple will be very happy.

If it's the only, or main, root, well ... Suggest you have a Certified Arborist or two out for an on-site evaluation. They can evaluate the existing root system by several none-destructive means, among them an air spade.