Captcha,jsch leading to error page

after solving captcha or just after js challenge cloudflare makes POST request to ‘/’ on origin
instead of GET and it lead to 403\406

403 because cloudflare does not pass CSRF header when making POST request
406 because server expects / GET instead of / POST

how to solve this ?
i am struggling with this about a month

please do not advice disable CSRF and make POST / to be correct response
from my point of view origin server is ok and i need to adjust cloudflare itself or use another solution

so i found out that sometimes POST request is working and sometimes not
the root cause seems like origin header so user should not reload page when it has cf_ parameters
looks like all good if these params are absent and page could be reloaded without 405\403

but in anyway it is very strange that sometimes origin does not receive POST after captcha and sometimes i see some 403 errors on NGINX logs - access log