what is your testing setup? I have tried oacs-head with the following settings:
UseHtmlAreaForRichtextP 1
htmlarea_p (in form_elements of message/post.tcl) not specified
and
UseHtmlAreaForRichtextP 0
htmlarea_p (in form_elements of message/post.tcl) not specified
and
UseHtmlAreaForRichtextP 1
htmlarea_p (in form_elements of message/post.tcl) 0
with forums/message-post, and it works as expected without problems. The installation has no local modifications...