My script keeps crashing in Busybox 1.27.2, so after some debugging I've discovered this behaviour:

# sh
# XXX=-1
# echo $((0-$XXX))
sh: arithmetic syntax error
# echo $((0- $XXX))
1

In GNU bash 4.4.12(1)-release the same code works:

# sh
# XXX=-1
# echo $((0-$XXX))
1
# echo $((0- $XXX))
1

It seems to be general for expressions even without variables like $((0--1)) with those results:

# echo $((0--1))
-sh: arithmetic syntax error
# echo $((0-+1))
-1
# echo $((0+-1))
-1
# echo $((0++1))
-sh: arithmetic syntax error