For anyone who upgrades to 6.3.x and suddenly starts seeing issues with collection fields that didn't exist prior. The error you will receive is "The field 'StartsWithSpace' doesn't exist within this collection" but since the field name doesn't start with a space in the error message, it can be a little hard to debug.
This works without issue in 6.2 and earlier. Checking the help document says that fields cannot contain a dot or bracket, but no mention is made of spaces. This only appears to affect fields that begin with a space. Ending with a space or containing a space elsewhere works as expected in testing.
I have spoken to BP support and they have advised that it was an unintentional break in 6.3 and should be resolved in 6.4.1 expected to be released in mid-November.