Walrus `SyntaxError: assignment expression cannot be used in a comprehension iterable expression`

What could be the reason that they disabled the use of walrus here also,

from itertools import *
total = ''
print([total := total + i for i in islice(x := 'ElonMusk.txt', len(x) - 4)])

gives the error described in the title of this post.

have to instead use,

print([total := total + i for i in islice('ElonMusk.txt', len('ElonMusk.txt') - 4)])

just found out there is a paragraph for it in the pep.
some issue with symbol table analyser.

one can always do something like this,

y = islice((x := 'ElonMusk.txt'), len(x) - 4)
print([total := total + i for i in y])