If you're confident that it is indeed bash executing your script, you can explicitly turn on brace expansion (expansion of {...} expressions) as follows:
set -B # same as: set -o braceexpand
Make this your script's first command after your shebang.
(Conversely, set +B (set +o braceexpand) would turn brace extension OFF.)
Conceivably, your system is configured to have brace extension turned off by default.
