I'd check the esoui source code online at github and search the code for achievement or tooltip at the file which shows the tooltip.
https://github.com/esoui/esoui/tree/master/esoui
e.g. if you are in the inventory and hover the mouse above an item it should be some tooltip in the inventory, so check the ingame/inventory folder for files about tooltips etc.
https://github.com/esoui/esoui/tree/...game/inventory
Or just use the search at the top to search for wordings, function names etc. you know of being used in the consense. Maybe just check the itemtooltip files and check which functions are able to add lines -> scan for those in the total code.
Maybe it's the achievementtooltips file:
https://github.com/esoui/esoui/blob/...nttooltips.lua
Or this file here:
https://github.com/esoui/esoui/blob/...ltipstyles.lua
Check for e.g.
ZO_TOOLTIP_STYLES =
...
achievementDescriptionIncomplete
It could be the red line? Search for the usage of ZO_TOOLTIP_STYLES[achievementDescriptionIncomplete] this text/part and maybe you'll find where it get's added.