This is a known bug in the Blizzard unit frame templates that affects all addons that create custom unit frames.
The easy solution is to create a macro '/focus' and put it on your action bars somewhere. Target the person/mob you want to focus and click the button, done.
The other option is to use the slash command '/focus target' and/or '/clearfocus'
Hopefully there will be a workaround to this some day.... /sigh
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