Note that Acer japonicum is not the same as Acer palmatum though both are often referred to as Japanese maple.
I have also seen dieback from tangled roots strangling each other. I have also seen as many examples of root systems sorting themselves out by natural selection of more productive roots and changing sap flow through fused roots.
With those tangled roots exposed to the air the tree has no choice but to work with the ones below soil level. Planting a few inches deeper can allow the tree opportunity to send out new, better formed roots that will eventually replace the restricted tangle below. You'll need to help by providing best conditions for that to happen - plant deep enough to allow it, make sure the soil does not dry out until new roots have established and reached out and down to deeper permanent soil moisture.
That approach is not guaranteed, just less intervention and hope for the best.
Alternatives include:
Nurse the tree through summer and next winter then bare root and do major root work to sort out the best root system possible. Pot and provide good care while it recovers then plant out the following year. Lots of work and still no guarantees but maples are very resilient and can survive major root reduction.
Snip off some of the worst roots you can get to now. Less invasive and still no guarantees.
Ground layer - either in the pot this summer or in the ground but in ground you'll need to provide constant moisture for the new roots to develop.
All things considered it is much better to choose trees with better attributes right from the start though I know sometimes choices can be limited. While we continue to purchase sub-standard trees growers will continue to produce similar stock because that's easier and quicker than producing good ones.
