I've installed a fresh 4.8.1 wordpress and your latest metabox plugin 4.12.2
There are no other plugins or other metabox extensions. I have followed docs and added the metabox and a text field. All works up to here.
However, when I try to save new text in text field, it doesn't save. Always stays blank. I have tried with other field types as well, and it doesn't save either.
WP_DEBUG true reports no errors. Everything else in page saves except metabox fields.
Was there an update recently that could have affected this?