I hope I can describe this…
I use my phone for work and have a list of vendors I use frequently. Recently one of those vendors stopped showing up in my text log.
If I need to text them I have to use the search function. When I do this it brings up my text history with them but if I go to recent texts they are not there. I’m sure I inadvertently toggled something but I can’t find what it may be.
Any ideas?
There is a text stetting in which texts can be separated into “known” and “unknown”. If this vendor isn’t in your contacts, their texts may be being shunted to the unknown list.
When you’re on the message app, if you see a back arrow from the main list of messages, you may have enabled that filtering system.
Good suggestion but I went to the message app and the filter unknown wasn’t selected.
Must be something else. 
Did you possibly put the contact in question on ignore? This could lead to the circumstances you describe.
And for the record, are you using the default message/imessage app, or a different option?