Issue with tables: asciidoc & grids/borders

Hello,  I found an issue with tables.
Using asciidoc markup (in)  table cell allow one neat feature: nested tables.
But the grid style of the sub-table get's messed up. I tried all the possible combinations here is the result:

none - none    OK
none - cols      OK
none - rows    OK
none - all        OK
none - undef  OK

cols - none    ERR (cols)
cols - cols      OK
cols - rows    OK
cols - all        ERR (cols)
cols - undef  ERR (cols)

rows - none    ERR (rows)
rows - cols      ERR (rows)
rows - rows    OK
rows - all        ERR (rows)
rows - undef  ERR (rows)  

all  - none  ERR (all)
all  - cols    OK
all  - rows  OK
all  - all       OK
all  - undef OK

undef - none  ERR  (all)  
undef - cols    OK  
undef - rows  OK
undef - all       OK
undef - undef OK

As you can see in some cases the sub-table inherits its "grid" style and don't sets it correctly.
I'm betting on my luck that this is a trivial fix.
All the "frame" combinations ("all", "topbot", "sides", "none", and undefined) work properly !
Th ank you.

Asciidoctor 1.5.5 [http://asciidoctor.org]
ruby 2.4.0p0 (2016-12-24 revision 57164)
Mozilla Firefox 50.1.0
Chromium 55.0.2883.87