In 2020 the German government lowered VAT in a reaction to covid from 19% to 16%.
On the first day Amazon had reduced thousands of products by exactly 19% and only increased by 16% after a few hours. AFAIK they did honor every sale during that time.
I can totally see that happening. Who develops systems with short term VAT changes in mind?
Aldi iirc just kept the old prices and simply gave a 3% discount at checkout