Yes, flash does take a little more care. Flash is not fully point&shoot, we do have to think just a bit about what we are doing.
Frankly, I have never noticed any issue at all with the flash in vertical orientation, and speedlights have been built that way for a few decades. Post a couple of pictures, same scene with horizontal and vertical
WITH EXIF, so we can see what you see. If shooting DX with a FX flash, it is a good bit wider anyway (only a couple of flash models switch zooming with DX).
But yes, Auto FP is Auto FP, and FP triggers on and off above the sync threshold (1/200 or 1/250 second) ... which is the meaning of Auto (what you told it to do). If faster than sync, it switches to be continuous light at 20% power capability. If not faster, it is regular speedlght mode. This is a big difference and kinda important to know, and again, you are the photographer, and it is your job to know what you are doing.
Metering is still TTL either way, should meter OK, if it can, but FP is only 20% power capability, which is a big difference. Do you watch the blinking Ready light to show when it does not have sufficient power for your situation? Then you would know what to expect. You can have it beep at if you if you don't watch.
FP is like Auto ISO, it is best to turn it off if you do not want it coming on.
Or, you can change A mode to be P mode, then camera will stop down all the way to f/22 or f/32 before switching into FP flash mode. Wedding photogs call P mode Professional mode, because they can move indoors to outdoors without thinking about what they are doing.