In addition to a description of the bug you are reporting, please provide:
Device Type: phone
Device Model: Google Pixel 9
Device Operating System: android
Diablo Immortal App Version: newest as of 4-3-25
The behaviour of the Survivor’s Bane rewards changed many, many months ago.
You no longer receive Mystery Legendary Item containers (that would stack up in the 4th Inventory tab) for completion of the Achievements tasks but, instead, you immediately receive a legendary item into the 1st Inventory tab. They just never updated the Achievements screen because, if you hover over the rewards, it still shows a Mystery Legendary Item container.
However, if you mean the rewards screen at the end of a run, that shows how many reward items you earned, those, whilst having an icon that looks like a legendary container, are not…
They are High Quality Equipment caches, i.e. it’s possible for them to have legendaries in, but not guaranteed.
Not getting legendaries, from containers that are not guaranteed to give legendaries, is not a bug.