If a Railing is associated to a Hotlinked Element (in A.pln), it will work correctly in the file where it was created (in A.pln). In case that you place this file in another project as a Hotlink (in B.pln), the Railing will be missing.
Railing elements should not be associated to Hotlinked Elements, but currently it is possible. We do not recommend to use this workflow in the meantime, because it can lead to missing elements.
do not place associative Railings on Hotlinked Elements, use Static Railing instead
include the Railing in the 1.mod, instead of placing it in the A.pln.
NOTE: This issues has been fixed in the Updates of ARCHICAD 21 (build 60xx). Click here to download the latest Upadate.