RESOLVED
We talked to Bois Efrady, Microsoft PBI R expert, and was informed that the root cause of this problem is that currently the URL retrieving is blocked in PBI R scripting. So a work-around is to encode PNG into Base64 string and then decode it into readPNG(). Here is the sample which displays a small Yen sign.
library(png)
library(base64enc)
tempNamePNG = "tempPNG.png"
#here is the string (copy-pasted manually after convertion at https://www.base64-image.de/encode)
oneLine_cha_data_back <- "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"
#decode string
binDataBack = base64decode(oneLine_cha_data_back)
#write decoded to png
to.write = file(tempNamePNG,"wb", raw = T)
writeBin(binDataBack,con = to.write)
close.connection(to.write)
#
img1 <- readPNG(tempNamePNG)
grid::grid.raster(img1)
Hope this helps.