"Anyone else encountered this ? Pierre ? "
That's a new one.
😞
I never buy postage at the Post office in any case and always affix the stamps on the envelope and packages before heading to the PO.
For GST/HST registered sellers, it does not really make a difference as the GST/HST charged today becomes an Input Tax Credit (ITC).
For non-registered sellers, it would hurt.
Canada Post had a similar bug in their program about four/five years ago, following a major upgrade. It affected the fuel surcharge which could not be prepaid with stamps. I met with the CP area supervisor who acknowledged the programming mistake. It took Canada Post one full year to correct the problem.
Patience!