I can confirm this behavior on Dozer 5.5.1. I've opened a ticket:
https://github.com/DozerMapper/dozer/issues/195.
The workaround seems to be to name the field in the mapping. Strangely, case doesn't seem to matter. For example, with the following mapping, the "mystring" field will not be mapped:
<mapping>
<class-a>UpperCaseBean</class-a>
<class-b>LowerCaseBean</class-b>
</mapping>
However, with the following mapping, the field "mystring" *will* be mapped:
<mapping>
<class-a>UpperCaseBean</class-a>
<class-b>LowerCaseBean</class-b>
<field>
<a>mystring</a>
<b>mystring</b>
<field>
</mapping>