Warning for \isotope command about chemmacros HOT 1 CLOSED

You are using this within \ch{} (from the package chemformla). Outside of it you don't get warnings.

Inside of \ch{} numbers are treated as stoichiometric factors or subscripts:

\ch{1/2 H2O}

That fact that your example doesn't give errors is merely by chance. In order to give the correct results here you need to hide the numbers from the parser by surrounding the inputs with "...":

\ch{\isotope{U} ->[\textalpha] "\isotope{234,Th}" ->[\textbeta] "\isotope{234,Pa}" ->[\textbeta] "\isotope{234,U}"}

from chemmacros.