look for the difference in these two lines:

[[M: 37 B: 5 C: 4][][M: 100 B: 14 C: 11]...
[[M: 37 B: 5 C: 4][, ][M: 100 B: 14 C: 11]...

No problem for me. I added in "Your example test cases":

assertEquals("[[M: 9991 B: 1427 C: 1110]]", Carboat.howmuch(9990, 10000));

and the tests passed. But if I put

assertEquals("[[M: 9990 B: 1427 C: 1110]]", Carboat.howmuch(9990, 10000));

which is false because of 9990 instead of 9991

I'm getting

expected:<[[M: 999[0] B: 1427 C: 1110]]> but was:<[[M: 999[1] B: 1427 C: 1110]]>

the extra brackets which you found weird are there to show where the strings are different! Hope that will help you!

Java tests are correctly written but for a reason I don't know the output of square brackets in CW is weird:-(

besides Kata Instructions state: howmuch(1, 100) => [["M: 37", "B: 5", "C: 4"], ["M: 100", "B: 14", "C: 11"]]

Note in "Your example test cases" and "Your solution" that in Java the function returns a string which kind of mimics an array.
In "Your example test cases" you could read:

assertEquals("[[M: 37 B: 5 C: 4][M: 100 B: 14 C: 11]]", 
				Carboat.howmuch(1, 100));

which, as you can see is correctly written.

Many Codewarriors passed the kata in Java without problems. I just tried myself and everything went fine.

The general workaround is to write faster code.
Although for this kata, I can't really imagine an answer which is too inefficient.
Infinite loop/recursion is more likely.

Could you post your code here? (Mark it as spoiler)