BUG: missing song covers in offline mode on android

[ZRyzner]

Description
I noticed that when I listen in offline mode, some tracks downloaded on my device doesn't have a cover (see attached screenshot). If I go online cover will be updated. But next time when I listen offline some covers are missing again. I wonder why? The cover should be included in the mp3 file, right? Even if it's not - why some of them are downloaded while others aren't?

What are the steps you took to get to the error/issue?
1. Open Spotify on Android
2. Make sure you're offline
3. Select a playlist with downloaded tracks
4. Check play queue to see if all tracks have covers

What I expected to happen
I expect that all tracks downloaded in my device will have covers available even in offline mode

Did you restart your device when troubleshooting in section B?
Yes

Did you read the connection troubleshooting tips?
Yes

Your device
Motorola MotoG 4G

Device’s Operating System
4.4.3 KitKat

What type of account do you have?
Premium

(FYI for the below points: Android devices tend to refer to the internal memory as "SD Card" and the SD card you inserted yourself as "External SD Card")
Space left on your device
1.5 GB

Space left on your SD card (if applicable)
N/A

Space left on your External SD card (if applicable)
25 GB

Is your device rooted?
No

Are you using a custom ROM, if so which one, exactly?
No

Did you set your external SD card as storage location?
No – app is installed on the device and offline music is stored wherever you have the most free space available as far as I know

Did you manually install an older version before installing the latest version from the Play Store?
No

Do you have the same issue when switching between WiFi/Mobile Data (If applicable)?
Yes

If you said "No", when does the issue not occur?
N/A

My mobile Spotify version
1.3.0.423

My desktop Spotify version (if applicable)
N/A

My mobile provider and country
Vodafone CZ

My username
1218955311

Do you have a Spotify (non-Facebook) login?
Yes

Are you logging in with Facebook or with your Spotify login details?
Both – there’s no difference in relation to this bug.

Do you have any screenshots you can attach to more clearly explain your issue?
See attachments