some of the threads which are active right now are all about the same issue but it seems your case is a little different.
It is possible that with 7.5.0 a newer version of the Antivirus/Antimalware engines was released and pushed to your MWG, which could explain why you see a different behaviour. While we could use this thread to collect the observations of other customers (e.g. did you notice a similar behavior?) I recommend to file a Service Request with Support and provide them with a handful of sample files and a feedback of your MWG installation. They will be able to replicate the issue and see if this is really a problem with the newer version or maybe independent. Also they are able to escalate the false detections if required.
Note: Please exactly state that you saw increasing the number of false positives after the upgrade of MWG. If you simply state "false positives" you might be send to labs directly, which is not the group you need to talk to.
For other customers could you please summarize how you detected the "400%" increase? Maybe by looking at some dashboard graphs (which ones exactly?)? This would make it easier for other people to quickly look onto their installation and see if they have a similar problem or not.
Thanks for the response. For the other customers investigating a possible similar issue, I used a couple resources to get the "400%" increase statistic. Turns out it's actually a much higher increase. I based my statistic off the e-mail alerts I have being generated when Malware is reported. I used to get 1-2 of these a day. Immediately after upgrading to 7.5.0 I started getting 8-10+ alerts per day.
The Web Gateway also has a built in dashboard that illustrates this perfectly. Go to Dashboard -> Charts and Tables -> Executive Summary
From there if you turn off all other indicators and just leave on the "Blocked by Anti-Malware" indicator they might see a large jump following the upgrade. Here's what mine looks like after the upgrade I performed late evening on the 26th:
Notice the drastic increase...It never went above 25 in a day and after the upgrade it peaked over 175. We have very little traffic over the weekend which explains the 2-day decline after the increase.
These detections are all Heuristic detections, but no changes were made to the system aside from the upgrade itself.