Why this matters
The bug. lru_cache doesn't deep-copy. The first caller sees settings['timezone'] = 'UTC' (defaulting). The second caller — even with a different user_id — gets a hit if the same id repeats, and any earlier mutation is now baked in. Worse, if the caller mutates the returned dict, future cache hits return the mutated dict.
The fix. Either return a fresh dict each time (cheap, defensive) or use immutable types (MappingProxyType, frozendict).
Heuristic. Caching mutable values is almost always a footgun. Cache the *raw* fetch and re-derive on every call, or cache an immutable representation.
Review heuristic
Every closure that captures a value from a render or a setup phase needs an honest answer to the question: is this value allowed to change between when I closed over it and when I run? If yes, the closure is wrong; switch to a ref, a functional updater, or a fresh read.