Using first-name shortcode causes text display issues in mail

I’ve added the shortcode in a test newsletter like this: Welcome [subscriber:firstname | default:adventurer],

When sending it to two different emails of mine at the same time (both added to a test list in the same way, except one has a first name the other not), the one with first name works:

While the other has a three dots button included that needs to be clicked to show the following paragraph’s text:

Removing the shortcode and putting a general greeting for all circumvents the issue but is obviously not something I want to settle for.

Any help to solve this would be much appreciated, thank you.