Possible bug in bash

[address@hidden]

For the follow script
alias al=' '
alias foo=bar
al for foo in v
do echo foo=$foo bar=$bar
done
bash (version 5.1.16) prints foo=v bar=, while all other shells I tested (dash, 
ksh, zsh, and yash) all prints foo= bar=v.
Apparently bash substitutes foo for bar in line 3 because the previous alias al 
ends with a space. But it is unintuitive that the word after for is checked for 
alias.
According to the posix standard,
If the value of the alias replacing the word ends in a <blank>, the shell shall 
check the next command word for alias substitution; this process shall continue 
until a word is found that is not a valid alias or an alias value does not end 
in a <blank>.
But “command word” is not defined. It is ambiguous whether “for” in this 
context is a command word, or whether tokens other than command word is allowed 
between the first alias and the next command word.

The same is true for case
alias al=' '
alias foo=bar
al case foo in foo) echo foo;; bar) echo bar;; esac