I think there may be 3 reasons why you had such a large drop in merch:
1) You lost both of your league games since the last merchandising update (one a TV game), and you got knocked out of the Cup. During the previous week, which you are using for comparison, you won all three games. So, some of that fall is due to the weakening of the fan survey.
2) You started your team about exactly 1 season ago. I tend to think that new teams get a bit of a fan interest boost for a some time after they start. Something similar to a promotion bonus, but not as strong. I'm not 100% certain about this, and it's hard to get good data, as many new players don't track it (understandably so; even my obsessive compulsiveness didn't extend to tracking it back then).
3) When you bought a high priced foreign player, it changed the salary structure of your team, and now the highest priced player is foreign. My current framework for how merch works is this:
-The players on your team are ordered by their salary (salary rank)
-That order is used to calculate what amount of merchandise they can bring in. So, the 2nd highest paid player will always bring in less merch than the higher priced guy.
-For each player, their status as to home country, drafting team, time on team, U21/NT status, is considered. A home draftee status pushes the possible merch to the highest amount for their salary rank.
-Fan survey overall is also taken account here, scaling the possible merch.
So, when you bought a high priced foreigner, he pushed everyone down a notch. Now, your top salary rank guy is no longer a home draftee (Ratkovic), but the foreigner. And your fan survey from yesterday must stink. Hence, a big drop in merch.
Of course, I have zero hard data to support this framework. It does fit with many observations and anecdotes that I've come across thus far. So I find it a useful way to think about merch. But it could be that the calculation is completely different.