It is not recommended to rely on local-part case, but it is indeed case-sensitive.

And this is to avoid such issues that postfix supports address cleanup/canonicalisation before forwarding mails to dovecot.

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RFC 5321:

"Local-part     = Dot-string / Quoted-string ; MAY be case-sensitive

[…]

While the above definition for Local-part is relatively permissive, for maximum interoperability, a host that expects to receive mail SHOULD avoid defining mailboxes where the Local-part requires (or uses) the Quoted-string form or where the Local-part is case-sensitive."