I've run into this same bug many times, but in my cases it is always with the report link field and not the lookup field. I can fix it in a report link field with the following process, perhaps in some weird way if you performed these steps on the report link field, it would fix the lookup field?
1. Edit the form properties of the report link.
2. Whatever is selected for 'Base the display on the report' change to anything else.
3. Save, then repeat the same steps and change it back to what you want.
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Mike Tamoush
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