I think I reached my limit of patience... It's been a year now and I still don't know why it happens. 

1. I create a playlist on PC

2. I "download" it on my Android Phone. I do not switch Offline Mode on

3. Everything seems fine, let's say I'm restaring Spotify App, phone etc.

4. Suddenly some songs can't be played even though playlist is still marked as fully downloaded

5. (I keep download via 2g/3g off). I'm switching on my Mobile Data, the songs that could not be played are played via 3g

6. I switch mobile data off. I switch Offline mode On

7. Again some more songs are not available. After restarting Spotify, those songs become fully (grey colour) not available

8,. I have to connect to Wi-Fi again to re-download the songs

9. -  everything repeats itself unless I play the songs while being connected to Wi-Fi, it seems(!) like the only thing that makes songs permanently downloaded

Of course between those steps is usually 1-2 days, not 30 days, before anyone says that songs need to be redownloaded every 30 days..

Anybody else experience something similar?