Hi.
I found next.
It happens when both plugins are active - CleanTalk and Akismet.
CleanTalk ct_contact_form_is_spam() function is running first in 'contact_form_is_spam' filter.
CleanTalk processes form and returns TRUE or FALSE, as needed in code in grunion-contact-form.php, process_submission() function, line 1335.
Next JetPack is_spam_akismet() function processes same filter because Akismet is active too. This function expects form array as parameter.
But WordPress passes previous filter function result - TRUE or FALSE from CleanTalk ct_contact_form_is_spam() function.
See plugin.php, apply_filters() function.
It's for current CleanTalk priority.
When I change CleanTalk priority to 11, situation is reversed.
CleanTalk runs after JetPack and recieves JetPack result (0 or 1) instead of form array as parameter.
IMHO the main problem is that both of our functions are intended for single using.
What solution can be best, what do you think?