FM is linear and FM Amount is pitch independent => at higher pitches the
impact is weaker
=> timbre is pitch dependent.
It's also possible to keep the timbre independent of pitch by making FM Amount be proportional to the carrier frequency:
FM Amount = 0
Mod 1: src: maximum; amnt: +59 (or whatever, must be >0); dest: FM Amount
Mod 5: src: keyfollow; amnt: +45; dest: M1 Amount
If you want to modulate FM ammount, modulate M1 Amount, or exchange in M1 maximum by sth. like LFO, env., modwheel,...