uipanel position jumping randomly

On 2021b, the following code runs with two different behaviors

Either the panel has the correct position (first picture) or it is too big (second picture)>
f = figure('PaperUnits','centimeters', ...
'PaperSize',[21 29.7], ...
'PaperPosition',[0 0 21 29.7], ...
'PaperPositionMode','manual', ...
'PaperType','A4',...
'Units','centimeters',...
'Position',[0 0 21 29.7],...
'Resize','off',...
'MenuBar','none',...
'ToolBar','none');

page_margin = 1.5;
pos = [page_margin page_margin 21-2*page_margin 29.7-2*page_margin];
main_panel = mypanel(f,pos);

function p = mypanel(parent,pos)
p = uipanel(parent,...
'Units','centimeters',...
'Position',pos,...
'BorderType','none',...
'BackgroundColor','w',...
'ShadowColor','w');
end

[Martin Privat]

Here is what the problem looks like on two runs of the code

Answer 1

[Martin Privat]

Many thanks to Daniele Sportillo for finding a solution, this seems to do the trick:

f = figure()

pause(0.1)

set(f,'PaperUnits','centimeters', ...
'PaperSize',[21 29.7], ...
'PaperPosition',[0 0 21 29.7], ...
'PaperPositionMode','manual', ...
'PaperType','A4',...
'Units','centimeters',...
'Position',[0 0 21 29.7],...
'Resize','off',...
'MenuBar','none',...
'ToolBar','none');

page_margin = 1.5;
pos = [page_margin page_margin 21-2*page_margin 29.7-2*page_margin];
main_panel = mypanel(f,pos);