If I have a autosign.conf that already contains hosts i.e:

.test1a.local
.test1b.local

I've noticed that foreman will append a hostname on to the last line instead of creating a new line. i.e. if building a host host1.test1c.local then the autosign will look like

.test1a.local
.test1b.localhost1.test1c.local

I've noticed that adding a couple of blank lines below the existing hosts mitigates the issue.

I cant reproduce this...... could it be that you didnt add an EOL (end of line) when you added another host?

Could be, though foreman should look for a new empty line before appending the host IMHO.