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Magnetism is one cause but I have been told by more than one industry person that if the watch (the balance spring specifically) is magnetised, the watch will always run faster, not slower.
I had a Seiko with a 6R that ran really well, until it suddenly didn’t. That started running a lot slower, and the watch wasn’t magnetised as far as I could tell - at least, using a degausser made no difference, and being a divers watch it had a degree of magnetic resistance built in. Seiko fixed it under warranty, which I believe for the 6R is simply swapping the whole movement out for a replacement one.
Ricochet
Regal
You should have a stamped warranty card with the purchase date inside the box.
Even if not, the proof of purchase might be enough - worth an email to service [at] seiko.co.uk and see what they say.