When capitalization for names and addresses is enabled in Constituent data hygiene, editing a household to use a custom name causes the word "and" to be capitalized. This also happens when adding an alias to the household or an individual.
This functionality is properly updating and capitalizing what it deems to be names entered into the name field. The correct solution here is to use the name format functionality to append names in this manner or manually edit the names to remove any unwanted capitalization after the initial entry.
Steps to Duplicate
[Prerequisite, go to Administration > Global Data Entry Settings > Constituent data hygiene > Edit > Mark the checkbox next to Correct capitalization in names and address fields > Save]
1. Go to a household record > Edit household > select custom name 2. Enter a custom name using the term "and" > Save 3. Notice that the word and becomes capitalized when saving.