Diki and others, this is what it says in the FP-50 Owners Manual:
"When Split Play is off, chords are recognized as follows.
While the damper pedal is not pressed, the FP-50 recognizes a chord when 3 or more keys are struck.
While the damper pedal is pressed, the FP-50 recognizes a chord when 1–5 keys are struck."
So it sounds like these have the "improved" piano mode introduced on the BK's (which they are contemporary with.)

One of the things I really like about the Roland style pianos is that Bass Inversion can be configured as a MOMENTARY function. When you hold the pedal down, it's active and the bassline follows the lowest note played. When you release the pedal, the light goes out and the bassline follows the root for any played inversion. This makes it a LOT easier to play the odd slash chord in an otherwise vanilla score. On Roland's arrangers, it takes one push to turn Bass Inversion on, and another to turn it off. Hard to do accurately while playing two chords on successive beats!