But the question is what will happen when 50Ω antenna, which has RL od 30dB, will be connected to this amplifier.
I think that in this particular question RL of 30dB probably means that antenna is almost 50 Ohm, for example 54 Ohm.
My understanding is that RL of the amplifier (4.65dB) represents its s22 and...
when it is connected to 50 ohm load, RL=-20*Log10(Abs((Zl-Zs)/(Zl+Zs)))=4.65dB
Your amplifier's Zs=63+i*80, but calculation also includes load's Zl=50.
..and RL of the antenna (50dB) corresponds to antenna's s11.
Antenna's s11 relative to what? I think relative to characteristic impedance Z0=50 Ohm.
RL=30db probably means that it's impedance is not 50 Ohm exactly, but Zl=54 Ohm:
Zs=50, Zl=54 RL=-20*Log10(Abs((Zl-Zs)/(Zl+Zs)))≈30dB
Now, because s22 is much higher than s11 it dominates.
This is probably wrong assumption. As soon as we disconnected 50 ohm load, RL of amplifier is no longer may be calculated using Zl=50, we have an amplifier with open connection Zl=large number, and new RL would be almost 0, and reflection would be almost 100%. Amplifier's S22 remains the same: s22 = (Zs-Z0)/(Zs+Z0) ≈ 0.41-0.42*i; backwards from s to z: Zs=Z0*(1+S22)/(1-S22) ≈ 62.5 - i*80.1
To sum up,
amp + 50 ohm load: Zs=63+i*80 ; Zl=50 ; RL=4.65dB
50 ohm 50 ohm antenna: Zs=50; Zl=54; RL=30dB IMO, antenna's RL nothing to do with 63+i*80..