Maybe some code is modifying the uitable after the startupFcn executes, because the code you've shown should work.
Here it is working with uitables in figures (since uifigures can't be created in the forum environment):
data = rand(3,4);
n_col = size(data,2);
f_args = {'Position',[10 10 840 100]};
t_args = {'Units','normalized','Position',[0 0 1 1], ...
'ColumnWidth',repmat({200},1,n_col),'Data',data};
% a table with default ColumnFormat
f = figure(f_args{:});
t = uitable(f,t_args{:});
% another table, identical to the first, except with ColumnFormat 'long'
f = figure(f_args{:});
t = uitable(f,t_args{:});
t.ColumnFormat = repmat({'long'},1,n_col);
I have also confirmed that it works with uifigures in R2022a.
