To be able to use full and wide align, you must run a theme that supports it, this requires the theme to set an option and add some CSS.
Twentynineteen supports this, but if you’re running your own theme you can add support for it yourself. How to do this is written in this article.
Now about the bit of code you sent, setting 'align' => 'full' should be correct and set the default alignment to full. Maybe try recreating your block to see if it’s set to full then?
I’m experiencing this as well. I’m able to set the default alignment if I set up the block as part of a template but the block does not respect the default alignment if it’s added after the fact to a page.
When your theme supports wide alignment via add_theme_support('align-wide');, you still have to enable alignment for your block. Replace 'supports' => [ 'align' => false ] with 'supports' => [ 'align' => true ]. Unfortunately, this enables the user to change the alignment, but it does set the default align.
NB: Using 'supports' => [ 'align' => [ 'full' ] ] is not possible at the moment due to a restriction in Gutenberg.
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