Hi Germinal,
I'm meeting the same issue mentionned here.
Invoice Creation :

The invoice is correctly created
Clicking on "PDF" button does download the correct Invoice
Clicking in "Send by Email" send the correct PDF.

Invoice Update :

The invoice is correctly updated
Clicking on "PDF" button does download the od Invoice
Clicking in "Send by Email" send the correct updated PDF (!).

I have tried to modify different parameters on my cache system, thinking it come from there, even totatlly de-activated it. I even tried to make "Cache cleaner" (from regular lab) to empty "pdfinvoices" folder when clicking on it, but the issue remains the same.

Is there any way to force updating the PDF when clicking on "PDF" button ?
Is there two different folder for the PDF (as in email it is updated but not with the "PDF" button") ?

Thanks for any advice.
I have created an access for you if needed, just ask, I'll send it by PM.