Update: the update is now available: check out the Fire Emblem Heroes Updates page for all the details!
A few days ago, Nintendo notified players about a glitch impacting Arena Duels in Ver. 1.1.0 of Fire Emblem Heroes. Basically, if you participate in an Arena Duel, close the app mid-battle (or if it crashes), then resume using the bookmark data, you will not get the the extra score boost you’d usually get for using Bonus Heroes.
What’s more, your Arena Defense Reward score may also be displayed inaccurately (or not displayed at all…), though you do get the reward correctly in that case.
The good news is, the developers have found what caused the glitch, and will release a patch to fix it soon. In the mean time, make sure to avoid leaving Arena Duels mid-battle, if you want your Bonus Heroes boost, that is… (unfortunately, nothing you can do if the app crashes on you).
In order to apologise for this issue, Nintendo is offering 5 bonus Orbs to all players (even those who haven’t been to the Arena at all), available as a one-time log-in bonus. You can collect them from the Present List, but don’t wait too long before doing so: you have until March 23rd to claim them! It would be a shame to pass on 5 free Orbs, don’t you think?
Source: Fire Emblem Heroes (notification)