I have the impression that there is debate over the state flags of Alabama and Florida, which were both created in the 1890s (at least the red St. Andrew's cross was added then). Some say they are based on the Spanish Burgundy Cross. Others say the Confederate battle flag. And there are other theories, I think.
In this book, The Confederate Battle Flag, which seems decently scholarly, the author John Coski seems to say the two state flags were based on the CSA battle flag, but then doesn't make a particularly strong argument about it.
This letter, from the Attorney General of Alabama, states bluntly that the flag is based on a Confederate battle flag. But I'm not sure how good a source it is.
I've read that in the 1910s the United Daughters of the Confederacy said that Alabama's flag was based on the battle flag. Do claims about the Spanish Burgundy Cross go back that far? Or is that an idea that came up later?
It is curious that both states adopted the same basic design at the same time. As far as I can tell Alabama was first and Florida essentially copied the idea? The flag laws they passed don't say anything about the meaning of the red St. Andrew's cross. Do we have correspondences between the states, their governors, legislatures, etc, about the flags?
Basically I'm wondering if there are real historical sources that back up one or another theory. Do historians agree in general one way or another, or it is a matter of debate and conjecture?
EDIT, to add one more observation/question: The Spanish-American War happened around the time Alabama and Florida added the red St. Andrew's cross. Today is seems totally reasonable that Florida, and maybe Alabama, would honor their Spanish history. But in the 1890s?
We don't really have definitive evidence either way of what the two flag designers were thinking, so it's not really a question we can answer with absolute certainty. The Confederate Battle Flag explanation does seem to be the earlier one though. In 1917 National Geographic stated rather matter-of-factly that the Alabama Flag was based on the Confederate battle flag, and I'm not aware of anyone talking about the Cross of Burgundy that early.
Personally I'd say there are a few reasons to go with the Confederate theory over the Cross of Burgundy:
Alabama's flag was designed 1895, right when Confederate symbolism was seeing a bit of a revival in the South. Mississippi's flag for example was adopted in 1894. This is also right around when The Sons of Confederate Veterans and the United Daughters of the Confederacy were established. It'd be a bit of a coincidence if Alabama adopted a flag during that period that looked so much like the Confederate flag for completely unrelated reasons.
Francis P. Fleming, the designer of the Florida flag, was a Confederate veteran and a segregationist. He added the red saltire so that the state flag would look less like a "flag of surrender". That type of rhetoric strikes me as more in line with a Confederate origin than a Spanish one.
The legislation adopting Alabama's flag describes it as a "Cross of St. Andrew". Both the Confederate flag and the Cross of Burgundy represent St Andrew's Cross but we rarely call the Cross of Burgundy a "Cross of St Andrew" in English. That term however was pretty commonly used to describe the Confederate flag in the past.
In both Alabama and Florida, the Spanish period is more commonly represented by the banner of Castile and Leon than it is by the Cross of Burgundy. You can see that on the coat of arms of Alabama and the seal of the Florida Senate.
I don't think any of that is firm evidence for a Confederate origin but that's certainly how I'd lean, especially since most other southern state flags are either based on Confederate symbolism or date to the Civil War.