[Bug] \tcreports{boards,report-I.report-II} got broken with last longtable/kernel update

[alceu-frigeri]

Working on alceu-frigeri/starray#6
and testing some alternatives \tcreports start breaking badly with low level TeX errors.

After some backtracking, and making sure it wasn't any change in starray code, I nailed it down to using
\begin{longtable}
More precisely,

            \begin{longtable}
              {
                p{\UseConst{report.tabA}}@{\ }
                p{\UseConst{report.tabB}}@{\ }
                p{\UseConst{report.tabC}}@{\ }
                p{\UseConst{report.tabD}}@{\ }
                p{\UseConst{report.tabE}}@{\ }
                p{\UseConst{report.tabEx}}@{\ }
                p{\UseConst{report.tabF}}@{\ }
                p{\UseConst{report.tabG}}
              }

Whereas \UseConst is a wraparound \prop_item:Nn which is expandable (and supposedly safe).

Anyway, initial workaround: New Document Command \GetConst which uses \prop_get:NnNTF to store the value in a token list first. replacing the above 'code' with

   \GetConst{report.tabA}{\tabA}
   \GetConst{report.tabB}{\tabB}
   \GetConst{report.tabC}{\tabC}
   \GetConst{report.tabD}{\tabD}
   \GetConst{report.tabE}{\tabE}
   \GetConst{report.tabEx}{\tabEx}
   \GetConst{report.tabF}{\tabF}
   \GetConst{report.tabG}{\tabG}
            \begin{longtable}
              {
                p{\tabA}@{\ }
                p{\tabB}@{\ }
                p{\tabC}@{\ }
                p{\tabD}}@{\ }
                p{\tabE}@{\ }
                p{\tabEx}@{\ }
                p{\tabF}@{\ }
                p{\tabG}
              }

TODO: further testing and see if there are no side effects.

This is probably related to:
latex3/latex3#1460
(might be) lvjr/tabularray#474

NOTE: I tried to get a MWE of the issue (just declaring some property lists, and then using then as above... it works :/ so it is a cross-related problem (some other package...))