My Fightstick Drops Inputs When I Press Two Buttons At The Same Time

The pattern you have described rules out the switches, which is the good news, because it means you probably do not need to desolder anything.

A switch either makes contact or it does not, and it has no idea whether another switch is being pressed. If your individual presses are 100% reliable, every switch works. What changes with a simultaneous press is what the encoder board has to do: it scans its inputs in sequence, builds a report, and sends it. If two switches close within the same scan window, a board with a slow scan rate or a poorly written firmware loop can catch one and miss the other, or report them in separate frames far enough apart that the game treats them as sequential rather than simultaneous. One in five is a very typical hit rate for a scan-timing problem, because it depends on where in the scan cycle your press happens to land.

There are two things to check, in this order. First, the encoder's polling rate. Many budget encoder boards run at 125Hz or even 100Hz, which is an 8 to 10ms window, and genuinely too slow for leverless play where your two presses may be two or three milliseconds apart. Good boards run at 1000Hz. If your board is configurable or has a firmware update, this is where the fix usually is. Second, the firmware itself: several popular open encoder firmwares have had scan-loop fixes released specifically for dropped simultaneous inputs, so if yours is more than a year old, update it.

The thing to be aware of is that SOCD cleaning, which most leverless firmwares implement, is a separate feature that deliberately resolves opposing simultaneous directions. If your dropped inputs are specifically two opposing directions, that is SOCD handling working as designed rather than a fault, and it is a setting rather than a bug. For two non-opposing buttons, which is what it sounds like, it is the timing issue above.