I've tested the following code in R2019b but couldn't reproduce the warning which you got. It works well.
parfor ii=1:2
a = ones(2^31+ii, 1);
fname = sprintf("result_%d.mat", ii);
parsave(fname, a)
end
function parsave(file, var)
save(file, 'var', '-v7.3')
end
Is there any different from your actual code?