It seems to be right up the alley of what we are talking about, key repeats for the a key seem to stop being passed through, in some manner related to the the European accented characters pop up option.
Obviously I notice that the d key is not listed as a special, however random testing might have got one of the affected key to stop input and seemed like any key does it perhaps and you've tested with a vanilla JPanel and you don't get the issue.
The post doesn't say what version of jdk he is using, but suggests that doesn't affect anything.
Is your version on MacOS 10.13?
Can you run your test program but put system out into the key typed and key released methods to see if they are still triggered what key pressed stops being called?
Could you retest to see if only the set of characters z, c, n, a, s, e, y, u, i, o are causing the issue. So run the app and only hold down the d key and nothing else and see what the behavior is?
If this path yields no good results the next step is to modify the Canvas3D class and remove anything that refers to KeyEvent and AWTEvent, this is a bigger piece of work so I think it's worthwhile thoroughly thrashing these OS version based ideas first.