Conditional headers

I even checked if "Header" directive works when setup from "Rewrite" section in VHost settings (to exclude any potential issues related to .htaccess file): it did not work in OLS, but it does work in LSWS. Either this is intended, or there may be a bug in OLS.
Did you ever set a simple test header like:?

header set x-testheader "test"

I remember an issue with LSWS in the past where I couldn't set a header because LSWS wanted to get header append instead of header set. I just can't imagine OLS refusing to set a header?!
That's exactly what I have been trying to do for testing:
header set x-test-header true
OLS sends it normally, if I use the "Headers" field for a "Context" (besides an issue with "merge", that I've reported already), but it seems to completely ignore the directive in any field related to rewrite or respective .htaccess. And, as mentioned, I can confirm, that it is reading the .htaccess, because for the test sake I moved some of the writes from VHost settings to the file.