The r/f is probably gummed up with old grease. The focus mechanism pulls the swinging arm out against the return spring when the focus is racked out to a close-focus position. If the mechanism is sticky, the spring lacks enough strength to pull the arm back again once the focus is returned to the infinity position.
You'll need to remove the top cover and then the rangefinder itself to clean away the dried grease and apply something more useful.
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