cet-is » mrb

Taking as example lariatsoft.
To reproduce, set a new MRB area up, then...

cd "$MRB_SOURCES"
mrb gitCheckout lariatsoft
export PRODUCTS="${PRODUCTS}:/grid/fermiapp/products/lariat"
mrbsetenv
tr ':' '\n' <<< "$FHICL_FILE_PATH" | grep lariatsoft

The last line shows to me that there is no FHiCL path set up for lariatsoft.
The FHiCL files are correctly copied into ${MRB_BUILDDIR}/lariatsoft/fcl, just the directory is not included in the FHiCL search path.
Adding a line

fcldir product_dir

to lariatsoft/ups/product_deps has the expected effect of adding the (default-named) FHiCL directory to that path, and therefore it works as a workaround.
From comments spread in the product_deps files I infer that the expected behaviour is that fcldir is optional and required only to change the FHiCL directory name from the default fcl/ (that LArSoft and almost all their experiments in fact turn into job), and that it should be included (if present) in the path regardless.
In conclusion, it's not clear to me if this is a bug or the expected behaviour.

Note that one consequence (maybe the only one) is the failure of the unit tests (if lariatsoft had any, that will happen soon).