On Reset Era they´re saying 720p portable mode and 1080p TV mode, both 30fps
BandaiNamco´s Youtube page doesn´t mention it so like I said above, don´t know where they´re getting the info from, but I guess it makes sense?..dunno.
It generally doesn't make sense to force the resolution much higher than 720p in handheld, as it looks great on that small screen at that resolution and you typically want to avoid straining the hardware to the point that it becomes hot and the fan noisy, not to mention lowering the battery life.
I'll be quite impressed if this runs at a locked 1080p and 30fps, though. Given they're also claiming 4k 60fps on the PS4 Pro (I really can't see how that's remotely possible), I suspect all of the stated resolutions for these ports are the high-end of what will be targeted with constant dips via dynamic resolution.