There is a problem (and people report it). ISO_Next_Group works in a cycle (for example, 0-1-2-0-1-2) while, ISO_Prev_Group works acyclically (2-1-0, and no more changes). For the hint on how to fix, see: http://listserv.bat.ru/xkb/Message/215.html
Just to remind, this is a big in the code (Xserver/xkb/xkbUtils.c), not in xkb database.
fixed in HEAD, will push to branch if it's not completely broken
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