This is the result of expectations created by lack of education on patina. If one expects that patinas will continue to change, and the natural change creates a completely unique and beautiful finish, emotions of wonder and excitement are generated rather than disappointment. Patinas develop on nature's timetable rather than man's. A patina in transition may alarm an uneducated collector, but an informed collector understands this transition as fact and bides for its result. Most patinas from the foundry transform at such a slow rate that it is hardly noticed by the collector unless compared to a new bronze.
Some patinas change more rapidly depending on the oxidation factors surrounding the bronze.
Either way, change of the patina is is very natural and beautiful and dependent more upon the laws of nature than any control man can establish. That is why artists and collectors for 500 years have cherished works of art with rich and deep patinas that developed over time. Circa 1900 bronze sculpture with natural patina.