Well, what did you expect? You're relying on undocumented features.
Relevant sections from the manual:
Definition of 'name':
| name A word consisting only of alphanumeric characters and
underscores, and beginning with an alphabetic character or an
| underscore. Also referred to as an identifier.
Definition of shell functions:
| Shell Function Definitions
| A shell function is an object that is called like a simple command and
executes a compound command with a new set of posi‐
| tional parameters. Shell functions are declared as follows:
|
| name () compound-command [redirection]
| function name [()] compound-command [redirection]
| This defines a function named name. The reserved word function
is optional. If the function reserved word is sup‐
| plied, the parentheses are optional. The body of the
function is the compound command compound-command (see Com‐
| pound Commands above). That command is usually a list of
commands between { and }, but may be any command listed
| under Compound Commands above. compound-command is executed
whenever name is specified as the name of a simple com‐
| mand. When in posix mode, name may not be the name of one of
the POSIX special builtins. Any redirections (see RE‐
| DIRECTION below) specified when a function is defined are
performed when the function is executed. The exit status
| of a function definition is zero unless a syntax error occurs or
a readonly function with the same name already
| exists. When executed, the exit status of a function is the
exit status of the last command executed in the body.
| (See FUNCTIONS below.)
It's common knowledge that if you rely on undocumented stuff, your
code will eventually break, like it did now. It's not a regression
though, nowhere in the manual you'll find that colons are allowed in
function names.
So, fix your scripts, perhaps?
