Seems like it my sequence is a list of hashes and as explained by Daniel Dekany this post:
The original problem is that, someHash[key] expects a string as key. Because, the hash type of FTL, by definition, maps string keys to arbitrary values. It's not the same as Java's Map. (Note that to further complicate the matters, in FTL someSequenceOrString[index] expects an integer index. So, the [] thing is used for that too.) Now someBeanWrappedMap(key) has technically nothing to do with all the []-s, it's just a method call, so it accepts all kind of keys. If you have a Map with non-string keys, you must use that.
Thanks D Dekany if you're on stack, this ended my half day frustration with the ftl template.