The wait method will not return a new template unless it has fees at least fee_threshold sats higher than the current template, or unless the chain tip changes and the previous template is no longer valid.
A caller may not be interested in templates with higher fees, and determining whether fee_threshold is reached is also expensive. So as an optimization, when fee_threshold is set to MAX_MONEY (default), the implementation is able to be much more efficient, skipping expensive checks and only returning new templates when the chain tip changes.