Could be a bug but the more likely explanation is that the field doesn't update until you try to leave it. So you put in a wrong date first, try to leave the field, get error message, then type in valid date and save a pic before leaving the field.
Ding ding, this is the likely answer. I've seen this exact issue before on multiple sites and services, and it's always been found that the form expects 18+.
...along with calendar weeks being entire weeks, not bullshit like 5 days of CW 53 and 2 of CW 1. This matters when dealing with BI facts that continue through NYE.
Do you know how infuriating it is to view analytics by calendar week with two major dips around new year's because the week is split in two?
Bonus points: "Sure, you can set the week aggregation to consider weeks starting with Monday, but if you filter for the last X calendar weeks, you'll have the last week's sunday omitted from the stats and an orphaned Sunday before the first week yoh actually wanted."
Support international standards, you bloody imbeciles.