7/19 22:20:01.452 Loading add-on BagBrother
7/19 22:20:01.452 ** Loading table of contents Interface\AddOns\BagBrother\BagBrother.toc
7/19 22:20:01.452 Error loading Interface\AddOns\BagBrother\Tests.lua
7/19 22:20:01.452 Loading add-on Bagnon
7/19 22:20:01.452 ** Loading table of contents Interface\AddOns\Bagnon\Bagnon.toc
7/19 22:20:01.452 ++ Loading file Interface\AddOns\Bagnon\libs\libs.xml
7/19 22:20:01.452 Error loading Interface\AddOns\Bagnon\libs\Classy-1.0\Classy-1.0.lua
7/19 22:20:01.452 ++ Loading file Interface\AddOns\Bagnon\libs\LibItemCache-1.0\LibItemCache-1.0.xml
7/19 22:20:01.452 Error loading Interface\AddOns\Bagnon\libs\LibItemCache-1.0\Tests.lua
7/19 22:20:01.452 Error loading Interface\AddOns\Bagnon\libs\LibItemCache-1.0\Caches\BagBrotherTests.lua

World of Warcraft\Logs\FrameXML.log

It's intended that way, because it does not affect the user, and makes it easier for me to maintain unit tests.
But where can you see that log?

This issue has started appearing in-game for me. Any ideas?

Those are the new file loading errors, just means the addon is trying to load Tests.lua because it is referenced in XML or TOC but the file is not there. To avoid it, just create an empty file at the place with that name.