I've always wondered, if someone has a drink the week before getting, for example, their 10 year pin, do they not get the pin, or is there some kind of duolingo take-back?
Put it another way, is 10 years with a few lapses worth significantly less than 10 years completely sober? Of course not. It's still a huge achievement. So why punish yourself?
If you think it's worth lying about having 10 years, then why bother lying about having 10 years?
In practice, milestones are more for sharing one's experience than for attaining an award. And if you slip up, telling that truth is honestly more helpful for the other person struggling to stay sober than lying to them about having 10 years.