I haven't had any issues with the magazine catch on my fnx. I know at least 1 members here that was having the issue of the mag dropping was due to their grip, and was self corrected. I think there has been 1 other member that has had this issue off the top of my head and ended up sending it back to fnh, and I believe it was corrected by them. Do some forum searches and you should be able to find the threads to confirm.
I wouldn't even call it a known issue, because a small percentage of them have even had this issue. I really think a lot of it is people's various grips combined with an ambidextrous mag release, having 2 places to drop the mag increases the likelihood of hitting it by accident. A glock and an FNX are 2 completely different beasts, make sure the fnx or the glock is what you want. There are many other 40s that are more similar to the fnx (such as sigs, berettas, etc.) and same for glocks (xd's, m&ps, etc.). I find it odd that these 2 would be matched against each other. Figure out what you are looking for in a gun besides just a 40 and that might narrow down your choices a bit more.