Been postponing to report this for a couple of years, hence not much to mention about iPhone model or app version, since it has been affecting a couple of years worth of updates on different models.
While early days of new UI (the one that finally introduce dark mode) had maps during running sessions correctly displayed according to the UI, after some weeks this started happening: whether UI is set to bright colours, dark colours, training in daylight or nighttime, the map is always displayed bright. Pretty annoying IMHO. I considered this a temporary glitch, only to realise we’ve been dealing with it for years.
