The tag data is a part of the file for each song, not in some mp3tag library. So if it existed prior to the update to mp3tag, it is still there. A screen shot of your library page and one with the details of one file would help to confirm this.
As you can see in the (green) title, you read APEv2-Tags with your current configuration settings.
Please re-read the first answer from @ohrenkino and try his suggestion.
How does a screenshot looks like then?
Love your rush to reply me, guys, but I still have the problem. Been properly tagging my MP3 for years now, but it seems they are gone, but the weird thing is Windows Media Player "remembers" the old tags, but if I try to edit them, they dissapear!
Nothing weird about that.
The tag data is still there - as you have shown us with the screendump that shows that you read APE tags which have precedence over ID3 tags.
Switch off reading APE tags and all the tag data that you don't see at the moment will reappear.
But that has been told by numerous participants in this thread.
Yet, you refuse to show us a screendump of the current settings in Tools>Options>Tags>Mpeg and a dump of the extended tags dialogue read with these settings.