Technically, Texans are not “Southerners” (they are Texans) but they use y’all for both singular and plural. At least they did around Houston.
Houston has long had a bunch of Cajuns living there who don’t use the term properly, it not being actually Cajun, and their use has infected the Houston area.
I was born in Houston and lived most of my adult life in Houston. I never heard y'all used as singular in Houston or elsewhere in the state. The only time I have heard y'all used as singular is in movies where they are trying to talk Southern but don't know how.