Only Smart Playlists match between iPhone and iTunes for songs purchased on iPhone

Question

I purchase an album on my iPhone.  The music shows up on my iPhone in its Purchased playlist.  I start my computer and the music automatically downloads on my computer and is placed into the Purchased playlist under the iTunes Store on the left panel in iTunes on my computer.  I sync the phone to iTunes in which I have this Purchased playlist selected to sync with my iPhone.  And here's the rub:
From then on, when I plug in my iPhone to sync, it shows up as a device on the left panel of iTunes.  You click on it to show the folders below and, when you click on the phone's Purchased playlist on the computer (left panel under devices), it shows all the new songs I just purchased there in the purchased playlist.  But when you open the actual playlist on the iPhone, some of the songs from the new album are missing from the Playlist on the iPhone.  Again, ALL of the songs still show on the playlist in iTunes and ALL of the songs STILL show on the playlist in iTunes that is under the device iPhone listing in the left panel of iTunes.  So...even iTunes thinks that ALL the songs are still in the Purchased playlist on the iPhone, but when you open the iPhone, some of the songs from the playlist are missing.
I've even tried creating new playlists with different names and dropping all of the new album songs into this different new playlist and syncing this new playlist to the iPhone.  Even then, the same songs are STILL missing on the new synced playlist on the  iPhone.
There is only way that I can make all the songs show up on an iTunes playlist and on the same iPhone playlist.  This only happens if I create and use a Smart Playlist to specifically select those songs.  A Smart playlist always shows the correct songs in both iTunes and the iPhone.  But, again ONLY if it's a Smart Playlist.  If I make any regular playlist and drop the songs into it in iTunes, the playlist shows up on my iPhone but is missing some of the songs.
One other thing...I have no idea how this happens or what is causing it.  Sometimes, one or two of the missing songs will show up hours or days later.  Not sure if it's because I played them on my phone or on my computer iTunes and that caused some flag to trip that then allowed them to show up in the playlist from which they were missing.
Don't tell me to delete all music and resync all music or to manually manage my songs or to restore my phone.  I've done that so many times and it does not work.  I've tried unchecking the songs and rechecking the songs between syncs.  I've tried deleting the songs from all devices and redownloading.  Nothing works.  The only way I've been able to fix this is the delete all content from my phone and do a complete restore and NOT restore from my previous backup (treat my iPhone like a new phone with no backup available).  With 4000 songs and 100+ apps and podcasts, etc...that is WAY too cumbersome and takes to long to do everytime I purchase a song on my iPhone.
Again, this is NO ISSUE with a Smart Playlist...but only a Smart Playlist.  The work around right now is that I have a Smart Playlist that always shows the last 200 songs recently purchased.  That playlist is always accurate on both iTunes and the iPhone.

Answer 1

I checked again last night, and now iTunes matched the phone settings, saying zero space was available. I had nearly 20 gigs of "Other" data :/
So I erased all settings/data on the iPhone and restored from an iCloud backup from last week, and now everything seems fine, with over 15 GB of available space.