I came here to report the exact same issue! Everything was fine until recently. Perhaps the last WordPress update broke something? I am using the same versions as you.
Everything appears to be sending correctly and the response looks correct on the initial save, but the data is not saved.
/wp-admin/admin-ajax.php?_fs_blog_admin=true
Request
acf[field_5fc3aad04255a]: 10
acf[field_5fc7261aae126]: 2021
action: acf/validate_save_post
nonce: e31d46a48c
post_id: 4987
Response
{“success”:true,”data”:{“valid”:1,”errors”:0}}
The update sends the exact same data to the exact same endpoint and gets the exact same response, but it works.
