Gibbrish viewing some emails in Mail app

Samples of what a gibberish email looks like:
?M7???M50t??C??|?M|?p:?`5 7???M50t?Ш?ק?<?zױ??ej)܅??zۭ???ק?:ڞ?ެIܡا???{??[XY??Y[?KB??HY?[?HH?X??HHH?[?HH?H??Y?HKT?ܘY?H[Y?[???˃B?B??]?܈H?ۜ?Y???Y?ZY[??[??΃B?B?Q[Y??܈H?[?HH?H??Y?HX?H?X[^???H[???H???[Y??? H0?X?0?X?[?[Y\˃B?Q?]?܈H[??X??H???ؘ[?HHY??H?H?ܜ?[?[X???ۚX??H[?X???H??Y??H??Y?HK??][?HH?Y[?K?B?T?H?YY[?H???Y??[??XY???HXHY?X?p???HY???ۜ?Y?H[??[?H??[?H?X??p?ۈHH?X??H[Y??ܜ?Y[?K?B?B?KT?ܘY?CB?H^[??p?ۈH?H??HHٚX?

When I view it in the email provider’s website (namecheap’s service) it’s clear. This only happens on a handful of emails.

The email is in Spanish and the header from the email provider’s website (the clear version) contained:
Accept-Language: es-MX, en-US
Content-Language: en-US

I’ve also recently noticed a .pdf attachment that was corrupted on nextcloud’s mail app. This is a new installation on a fresh server.

nextcloud 14.0.1 (all setup & security checks passed)
PHP 7.1.22
CentOS Linux release 7.5.1804
mysql Ver 15.1 Distrib 5.5.60-MariaDB, for Linux (x86_64) using readline 5.1