Packs missing when launching the game

LegendSimms​  This is probably EA App-related, but beyond that, it's not always obvious why this happens.  If it's an intermittent issue that clears up on reload, it's especially difficult to troubleshoot since you can't predict it and also can't test a fix or workaround.  So I'd suggest ignoring it unless it becomes more frequent, or a reload doesn't clear it up, in which case I can offer some ideas.

Your mods and custom content are unlikely to have any effect at all.  What might have an effect is launching the game with the EA App in offline mode, if that's something you do on occasion.  Another possible trigger is launching from the Sims 4 shortcut rather than opening the App and pressing Play there.  But if you're doing this because it's convenient, it might not be worth stopping just to avoid the occasional reload.