Historically UriComponents has always encoded only characters that are illegal in a given part of the URI (e.g. "/" is illegal in a path segment), and that does not include characters that are legal but have some other reserved meaning (e.g. ";" in a path segment, or also "+" in a query param).
UriComponents has also always relied on expanding URI variables first, and then encoding the expanded String, which makes it impossible to apply stricter encoding to URI variable values which is usually what's expected intuitively, because once expanded it's impossible to tell the values apart from the rest of the template. Typically the expectation is that expanded values will have by fully encoded.
While the RestTemplate and WebClient can be configured with a UriBuilderFactory that supports different encoding mode strategy, currently there is really no answer when using UriComponents directly.