Acknowledgements section always gets line numbers in v6.3.1 [Which breaks arXiv submissions]

[zpleunis]

Using aastex631.cls the acknowledgements section always get line numbers (i.e., also without linenumbers selected as an option).

I was able to fix this by replacing \begin{internallinenumbers} and \end{internallinenumbers} with \ifnumlines\begin{internallinenumbers}\fi and \ifnumlines\end{internallinenumbers}\fi on lines 1240, 1249, 1296 and 1305 in the .cls file.