I have a custom field of type 'text' which allows multiple lines. When this field is rendered in the UI all newline characters are ignored and a single (long) line is rendered (issue list, issue details sidebar, issue boards).

For example I have a field customers that are affected by an issue:

<customer-id> - <customer-name>
<customer-id> - <customer-name>
<customer-id> - <customer-name>
....

IMHO formatting should be respected in the UI, otherwise the field value can be hard to read.