I've actually complained to Amazon about this, and they (allegedly) have a team working on it.
After only having my Echo Spot a few days, I set a recurring alarm for weekdays. All good so far.
But then as I was having a day off work, I asked Alexa to cancel the alarm for Tuesday. The response was "You don't have an alarm set for Tuesday". Well yes I do. Tuesday is a week day!
When I emailed Amazon, the person who replied agreed that it was an issue, told me that it had been sent off, and that I would get another reply when it was fixed. Needless to say, there was no further reply.
I emailed again and they just said "Oh sometimes things take a bit longer". Since then I've heard nothing.