The issue arises because the acm bibliography style would appear to contain \sc (and possibly other "old") font switching commands. For instance, the BibTeX function scapify is defined as follows in the file acm.bst:
FUNCTION {scapify}
{ duplicate$ empty$
{ pop$ "" }
{ "{\sc " swap$ * "}" * }
if$
}
However, the memoir class no longer recognizes \sc and other oldstyle font-switching commands by default. By "old", I mean LaTeX versions up to version 2.09, i.e., prior to 1995. Really old by now...
The fix? Load the memoir class with option oldfontcommands:
\documentclass[oldfontcommands,<other options>]{memoir}
Do be aware that warning messages will be produced whenever an old font command -- such as \sc -- is encountered.
\bibliographystyle{plain}with\bibliographystyle{acm}would be a good start. – Marijn Oct 17 '22 at 09:05memoirdocument class -- a fact you didn't mention in the main body of your posting. – Mico Oct 17 '22 at 09:43