I had this as well, but am not sure if my issue was the same.
My form was generated from within a plugin, via twig essentially. Although this looked like a caching issue, it wasn’t actually. I don’t understand all the fine details, but twig output is cached a little differently than page output. Apart from caching, I had to make sure my twig gets executed each time. Here you have my front-matter that made it working for me. YMMV.
never_cache_twig: true
process:
twig: true