Moving This to Bug Reports, because I do think this is something that's not working as intended.
The issue is how hearthings choose what to eat. In this instance, the setup is such that the hearthlings have sufficient food options, yet complain about them and eat the same thing repeatedly. I'm pretty sure its related to how hearthlings choose food. My guess is hearthlings path to the nearest container with food and randomly select what to eat, causing them to eat the same thing repeatedly, even though options are available to them.
Steps to reproduce:
Have enough Farmers/Shepherds/Cooks to support your town. In my case this was 6 Farmers, 1 Shepherd, 3 Cooks for a little over 30 hearthlings. Set your cooks to maintain all the different food types so hearthlings have a multitude of options.
Hearthlings to be happy with their food situation and no "On Eating the Same Thing Repeatedly" reports in the journal.
Hearthlings complaining about eating the same thing repeatedly despite having multiple food options. Notably, different hearthlings are complaining about eating different food types repeatedly.
My food storage is spread throughout the buildings of my town (in case that's relevant)
Both these pictures were taken at the same time. So at the same time that I have various food types available, they're complaining in the journal about repeated eating the same food.
Versions and Mods:
release-707 (x64) (no mods)
OS - Windows 10
CPU - i7-7700k
GPU - GTX 1070