I’m not familiar with WordPress using the title as a fallback alt attribute? Are you sure it’s not just in your theme?
I can’t say for certain wether ACF used to do this but I honestly don’t think it’s a good idea. consider the gallery field. Lets say you have 10 images in the gallery. If they all get the title as alt it’s completely useless for both search engines and text-to-speech etc. In those cases it would probably be preferred to keep an empty alt tag (But there should always be one!).
So in short, if your title would be applied to many images it’s not a good idea. If the title has nothing to do with what’s on the image it’s also not really a good idea to use it.
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