gmail : reply-to fail for first email from new mail address

My webapp let users send email where “from” address is generic (e.g. [email protected]) and “reply-to” is set to user email account.
The first time the receiver get one of these email and try to reply, gmail client systematically ignores “reply-to” and use “from”.
For next emails “reply-to” is correctly managed (even without save sender address in contacts!) and “reply-to” is used but for the first email “reply-to” is always ignore!
I’ve tried with many gmail accounts and this behavior occurs regularly.
Comparing emails headers from correctly managed email and wrong ones I saw they where exactly the same.
Anyone notice this strange behavior and know how to solve?