Nah we haven't.
My guess is that they wanted to go in different directions, seeing as Self Preserved is probably their most diverse album in that sense.
Some really heavy songs, some very mellow.
If that's the case though, it's a pretty lame reason.
They have always been a band of many genres, and one of their best qualities was that their music had a lot of variety, they could've easily continued making albums satisfying both the need for heavy but also more mellow music.