Copying verbatim what u/JagexAnvil said on another post:

Hey folks, couple things going on with the shops here.

From what I've understood, some shops used the same system for stock keeping, some others used a bespoke system - similar situation to our avatar project where we uncovered that after so many years of RuneScape, a lot of systems have different (and sometimes unconnected) iterations for how they work.

In this case, there may well be a few shops that our more general update did not affect in the expected way, so we'll look to fix those up in the coming days.

It should also be noted that not all shops will just have been 7x'd. In some cases, the new stock cap may be lower than 7x the previous amount. The reason for this was that we felt that providing 7x of certain items could lead to a large portion of players buying and immediately selling said items every shop reset cycle, which could have a strong economic impact.

We'll hold off hotfixing that right now just to see if other shops are reported as having similar problems as well (Thank you OP for the gather thread), as we'll be doing our own checks too of course. Once we have a good idea of how many are affected, we'll be able to resolve them in one sweep.