Due to the changes with daylight savings there is a bug where the field boss will spawn before the reset and you are able to get rewards from it even if there was no field boss on the day before. It does not count as that days reward either so you can get 2 field bosses in one day.
Hello @Maljfo,
Welcome to the Lost Ark Forums Community! We certainly appreciate the bug report. I will forward this along for you so a resolution will be available for you soon. Thank you!