Hey Mapoule,
It looks like your lightmaps are using the low res mips. Just to test if this is a streaming issue would you mind disabling texture streaming in your project settings then test the packaged game?
If this is the case make sure to enable texture streaming again since this is important for optimization. Try increasing your streaming pool size by entering the following cvar in the console ** r.Streaming.PoolSize ** this should be followed by 4000 or whatever your GPU can allow in VRAM. The default is 1000 (this is in MB) so most users can increase this amount.
If this isn’t a streaming issue we can continue from there.
Good day!
Ed