Mail disappear from AJAX webmail

[d3vnull]

Hello,
I have a strange problem.

On one Zimbra 8.6.0 account, when I receive some messages (seems random), I can just see them for 1 second and then Inbox still reports 1 message as read, but I can't see any new message.
If I change from AJAX to HTML webmail I can see every message.
If I come back to AJAX they are still hidden.

Tried to apply patch4, but problem persist.
Tried to upgrade to 8.7.9, but problem persist.

There's no filters on this mailbox.

I even tried to recreate the mailbox reimporting messages with imapsync, but it didn't solve!

What can I check? What could be the problem??