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