I know that there are some bugs in that game, but when you are buying more potions at apothecary shop, you can easily not get them if you buy it more at once. So that is another bug besides the graphic corruption.

Buying them too quickly results in you buying the next potion after the money is subtracted from your inventory but before the potion is added.

The best way to avoid this is to either play in ScummVM or wait a second between each purchase.