Indenting only 1 level on 1.5.x with threaded view

[cybertinus]

Hello all,

I've upgraded my Roundcube install from 1.4.11 to 1.5.1 and later 1.5.2, and since the 1.5.x line, the indenting is only 1 level anymore when you have enabled the threaded view. Something I have in every mail folder, because I find that a very useful view to group mail threads together. I've downgraded to the 1.4-line again, and upgraded that now to 1.4.13. The indenting behavior is as expected there as well.
I've made screenshots of a mail thread on a public mailing list, both in Roundcube 1.4.x and 1.5.x to show the difference and I've attached those screenshots to this post.

Can anybody give me a tip what I might be doing wrong in 1.5.x? Or did I hit a bug or something?