Why does NaH2PO4 become acidic in water? I thought this reaction would yield NaOH and HPO4- and NaOH is a strong base! Please explain what happens to the sodium ION. Does it accept a hydroxide?

An aqueous solution of NaH₂PO₄ is acidic because the H₂PO₄⁻ ion is a stronger acid than a base. The Na⁺ ion acts only as a spectator ion to balance the charge.
When you dissolve NaH₂PO₄ in water, it dissociates into ions.
NaH₂PO₄(s) → Na⁺(aq) + H₂PO₄⁻(aq)
The H₂PO₄⁻ ion is amphoteric. It can act as either an acid or a base.
The equation for its reaction as an acid is
H₂PO₄⁻ + H₂O ⇌ HPO₄²⁻ + H₃O⁺; ##”p”K_”a” = 7.21##
The equation for its reaction as a base is
H₂PO₄⁻ + H₂O ⇌ H₃PO₄ + OH⁻; ##”p”K_”b” = 11.88##
Since ##”p”K_”a” < "p"K_"b"##, H₂PO₄⁻ is a stronger acid than it is a base. This means that of NaH₂PO₄ will be acidic.