emails showing RFC2047 encoding

Hi support, I’ve been having a problem for a while now where any email sent from my site displays the RFC2047 encoding. This started after updating PHP from 7.4 to 8.1. I’ve tried several versions of PHP since non seem to correct this issue. A test email from WP Mail SMTP begins (edited to remove domain and IP):
[email protected]?=
X-PHP-Script: domain.com/wp/wp-admin/admin.php for IP address
Date: Mon, 15 Apr 2024 09:52:58 +0000
From:
Message-ID: [email protected]
X-Mailer: WPMailSMTP/Mailer/mail 4.0.2
X-Mailer-Type: WPMailSMTP/Admin/Test
MIME-Version: 1.0
Content-Type: text/html; charset=UTF-8

Any email from my site now includes this RFC2047 encoding. Any tips on how to resolve?

Thanks.