[NTG-context] Header bottom line vanishing after 87 pages

Hello good morning, I encounter a strange problem when compiling my project. The book has 252 pages, compilation runs without errors. But after 87 pages the bottom frame of the header is turned off. The \setupbackgrounds command is given only once in the preamble. — So I am curious what that can be. I tried to reproduce the problem by a simple 255 page loop, but there the issue does not appear. There is no difference between MKIV and LMTX, no difference in older version and yesteray evening’s version of ConTeXt. My question is whether anybody has an idea how to track down this issue. Kind regards Willi