Diamond used to supply monthly data (Top 300) and yearly data of Comic Shop sales - this was obviously more robust on capturing gross receipts by Marvel, but the data would lag on comic buyer purchases*. The ICV2 data that only shows a Top 50 has a few problems that matter less at the top (sure, #1, 2, and 3 on the chart might be transposed due to sampling issues, but they're all strong sellers and who is number one in a given month is less important than who stays a strong seller month over month), but definitely matter at the bottom**. Since we don't get nearly as interesting end of year data (which combines all the months' data) or have sensitivity at the lower ends of the charts, it's impossible to see if there are just shifts in purchasing habits that don't matter or if books we like are really starting to be in trouble.
*For example, most shops will base their orders for, say, Miles Morales 22, on how many sales they made of Miles Morales 18, 19 and maybe 20 (depending on when the order cutoff date is). If something happened in Miles 21 that caused a bunch of people to drop the title, it would not show up in the Diamond-style store sales data until issue 24 or a bit later, but it'd shop up REAL quick in the ICV2-style consumer sales data
**By only going to 50, we don't see a lot of books, and we don't know what ranking matches to what sales level. It's not impossible that some people that buy Miles Morales, a monthly book, only stop by their shop every few weeks, where ASM buyers might be much more likely to come in nearly weekly (since the book comes out twice a month plus the occasional special or tie-in). One could imagine that Miles is landing 30K sales for his book in his current ICV2 month and another 20K in the subsequent ICV2 month (big whatever on the specific numbers) which causes the book not to appear on the chart in either month, but the book has a very healthy 50K sales which Marvel would be happy with. It's also quite possible the numbers are the numbers and the sales on Miles Morales are tanking, but it's a little odd that they'd fall off so much right after an event.