[NTG-context] Weird background colors in autogenerated index

Dear All, since LuaTeXt 0.81 my back-of-the-book index (register) is generated with strange background colors. The index title is typeset in yellowish background and the left column of the last index page has a blue background, see the attachment. I've thought I reported it, but it is still there even in 0.95. It was fine in the version 0.80. Could this be somehow eliminated? Thanks, Jan