multirow with top-align functionality incompatibility issue?

I am working on a document, in Overleaf, using multirow in the usual fashion to make my tables look less crowded. I would like to vertical (top) align my multirow cells. However, I’m seeing unexpected behaviour.

multirow{2}{*}{textbf{2}} gives normal behaviour (bold ‘2’ centered vertically)

multirow[t]{2}{*}{textbf{2}} gives an error: “Missing number treated as zero. Illegal unit of measure (pt inserted).” Followed by Overfull errors, presumably caused the automatic insertion to fix the first and second errors. Compiled I get black boxes in my table with “[t]” printed, although it is top aligned.

I have tried to recreate this error in a new document, but I see only the expected behaviour! I have a lot of other packages in use and formatting on the document (I need to use a proprietary template) – are there any incompatibilities that I should investigate. Is there a common mistake I am making?

Previous (solved) questions have cited possible incompatibility with siunitx: one fixed with extra braces around multirow and one fixed with tablenum. I am using siunitx, but the errors I’m getting are different from those reported previously and the solutions did not work in this case.

Simplified code (Note: this works!! – the same code in my document does not.):

usepackage{multirow}
usepackage{siunitx}
sisetup{
   detect-mode,
   detect-family,
   detect-inline-family=math,
}
usepackage{tabularx}

begin{table*}
  begin{tabular*}{textwidth}{@{extracolsep{fill}}llrrrrrr}
    hline
    A & B & C & D & E & F & G & H 
    hline
    multirow[t]{4}{*}{textbf{1}} & i & 1 & 2 & 1.0 & num{-2} & 0.0 & 6.0 
    & ii & 1.0 & 103 & 04 & 1 & 0.4 & 10 
    & iii & 2.5 & 6 & 0.06 & 0.78 & num{-0.8} & 6.2 
    & iv & 1.3 & 35 & 2 & num{-10} & 0.06 & 1 
    multirow[t]{2}{*}{textbf{2}}& v & 80 & 1 & 0.44 & 0.80 & 0.22 & 2 
    & vi & 100 & 12.31 & 0.02 & 0.30 & num{-1.16} & 1 
    hline
  end{tabular*}
end{table*}