I have set up a custom Error Message Inbound Filter based on the title of the event (found in the event JSON), but errors matching the specified string are not being filtered.

Error Message Inbound Filters typically work by filtering errors based on their title, which is usually a combination of the event's type and exception.

However, if the error is a synthetic exception, we use the function name in the top frame of the stack trace as the event's title instead. Because of this, inbound filters will not work for the titles of synthetic errors. These filters only apply to titles that are constructed from the event's exception type and exception value. To verify if the error is synthetic, check the event's JSON for synthetic set to true under the exception.values[0].mechanism key.

To effectively filter synthetic errors, you'll need to use the event's exception type and/or the exception value.