[exim] Local variables tainted?

Hello,
I'm in the process of upgrading to new exim versions and thought I was
fully prepared for the new concept of tainted variables. But this has
left me rather confused:


$: exim --version
Exim version 4.94.2 #2 built 13-Jul-2021 16:04:57

(Debian Package: 4.94.2-7)

Warning: ACL "warn" statement skipped: condition test deferred: Tainted
MySQL server 'localhost/greylist/greylist/xyz'

I am using a MySQL DB for greylisting. The config variables are defined
like this:

"GREYLIST_DBSERVER = localhost/greylist/greylist/xyz"

This is then referred later:

warn set acl_m8 = ${lookup mysql{servers=GREYLIST_DBSERVER;\
GREYLIST_SUBNET}{$value}{result=unknown}}

Why would a local variable which is never set by any external input be
tainted?