Thank you for reaching out to Community!
I tested on my local site and the fields added to the card layout for the backlog will always add an additional line. Currently, it’s not possible to change this to show all in one line.
There was a feature request a while ago suggesting this, but it was closed informing that there was no plan to implement it on Jira Software:
However, there are two other feature requests where it’s currently on Gathering interest:
Please, click on vote and watch to receive updates about the features.
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