admin-ajax.php 404 (acf_invalid_nonce)

Ahoy,

As always, thank you for this absolutely awesome plug.

So on one of our sites today we ran into an issue where ajax requests being sent from an acf field (post field) were returning a 404 error during post search. The response being:

{"code":"acf_invalid_nonce","message":"Invalid nonce.","data":{"status":404}}

Flushing the cache cleared the issue for all users. Why might this be? And how might we prevent it in future?

Thanks in advance!