MH4U is not the only game having that problem. Gateway games in general have that problem and if you close the device and put it in sleep mode there is a -chance- that it might hang when you try to close the game, which makes the save completely corrupt. If you don't have a backup you're screwed. CIAs probably don't have this problem, but certainly don't quote me on that.
Here's another thread on the subject: https://gbatemp.net/threads/need-help-lost-140hs-on-mhu4.383474/
Personally I lost my 30 hour Persona Q save due to this bug. Afterwards I've been carefully taking backups all the time using SaveDataFiler and a backup script I made that automatically backups all saves on the SD card if it is inserted into my computer. You can find that script here: https://gbatemp.net/threads/need-help-lost-140hs-on-mhu4.383474/#post-5386979 However it only works on OS X, not on Windows or Linux (Linux compatibility should probably be quite easy to get).
I'm sorry for your loss man, there's nothing you can do about it. The save is completely gone unfortunately. Until Gateway fixes this problem (I've never seen or heard of it before GW 3.0) you'll need to backup saves. Often.
Here's another thread on the subject: https://gbatemp.net/threads/need-help-lost-140hs-on-mhu4.383474/
Personally I lost my 30 hour Persona Q save due to this bug. Afterwards I've been carefully taking backups all the time using SaveDataFiler and a backup script I made that automatically backups all saves on the SD card if it is inserted into my computer. You can find that script here: https://gbatemp.net/threads/need-help-lost-140hs-on-mhu4.383474/#post-5386979 However it only works on OS X, not on Windows or Linux (Linux compatibility should probably be quite easy to get).
I'm sorry for your loss man, there's nothing you can do about it. The save is completely gone unfortunately. Until Gateway fixes this problem (I've never seen or heard of it before GW 3.0) you'll need to backup saves. Often.