Thanks for the reply @ShanaBritt Yes sadly, I thought I had recreated 2 of 3 locators but in fact 2 were still 10.x (*.loc, *.loc.xml, *.lox). After re-creating the locators as 2.9 (*.loc, *.loz) style locators in the same file location and with the same name, the project at 3.0 will open without the 'Invalid Locator' message.
However, even after removing the updated file-based locators from the 3.0 project, upon a locator 'refresh' from the catalog pane, the file-based locators are re-added to the project, suggesting to me they are never actually removed from the aprx. As you note, they would have had to have been removed at 2.9. Maybe a 3.1 patch will fix this.
For me, I am using this as an opportunity to re-create new, clean 3.0 pro projects in a new directory location and then migrating data, folders, styles, etc. I am isolating my pro project used to create my address locators in an entirely separate location (separate from the home location). - hopefully this will keep things cleaner.