Mispelled words highlighted but suggested correct word only shows suggestion for words starting with same letter as mispelled word. It happened before with previous s9 randomly.
s20+ worked ok for couple of days, system update and all messed up again.
On this one can't even use neutral beta keyboard!
You could send this as feedback to Samsung via your Samsung Members App @MaggieF
Also maybe try out another type of keyboard which may serve you better in the interim.
Current Phone ~
Samsung Note 20 Ultra 5G SM-N9860_256Gb SnapDragon 865+
Samsung One Ui 3.1/ Android 11.