/usr/lib64/hevea/hevea.sty is kind of useless...
that's why it installs /etc/env.d/99hevea containing: TEXINPUTS=/usr/lib/hevea: which works in most cases but is not additive wrt our env.d system, hence if something else sets TEXINPUTS (eg ocaml) it becomes useless.
but you're right: it should be installed in a better place and we shouldn't use this hack I've removed the TEXINPUTS hack in 1.10-r1 and made it install hevea.sty in a suitable location