iPhone XS: s/mime signature doesn't contain the issuer (intermediate) certificate in signed emails

I'am using a valid s/mime certificate to sign my emails on my Mac Book Pro and my iPhone XS. When I sign the email from within the mail program on Mac Book Pro, the signature contains the certificate of the issuer (intermediate) of my certificate too. When I sign the email on the iPhone, the issuer of the certificate is not added to the signature. For that reason the email is displayed on the iPhone as not valid. The same problem in MS Outlook. The signed emails from Mac Book Pro are displayed correctly as valid. Why doesn't add the iPhone the issuer certificate to the signature?

iOS: 13.4

Signed email on MBP (intermediate certificate is included):

Signed email on iPhone (intermediate certificate isn't included):

You need to manually install the intermediate certificate itself on the iOS devices. I.e. it is not enough to just install your own S/MIME signing certificate on the device - you also need to seperate install the intermediate certificate.

A way to install the intermediate certificate is to have it as a P12 file in for example an email. Then you can tap on the P12 attachment and choose to install it.