After each of your plots, you should type ppl shaset reset. (This is overkill, but it will work).
E.g., from you script:
SET VIEWPORT A1
let WRF_Mar=Rain1984_March[d=1,x=28E:50E,y=12S:6N]
fill /nolabel/nokey WRF_Mar
!fill /nolabel/nokey/level=(0,1100,30)(inf) WRF_Mar
!ppl fill
go focean 5 white
go land 1 " " 1
label 38,6.6,0,0,0.19 @as Simulated Rain(Mar)
ppl shaset reset
!add this line
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