problem registration on 2 lists

Hello there,

I use the M4WP premium plugin.
My site has several forms, which all subscribe to 2 mailchimp lists.

But I have a problem, regularly I have users who are registered to only one of the 2 lists. It is not always the same one, it happens that they are registered on both lists.
I can’t solve this problem and I’ve exhausted all the solutions I could think of. So I’m contacting you to see if you’ve ever encountered this case and if you have any new suggestions for solutions.

Here is what I have already checked or tested:
– the 2 lists are checked in the settings on each form
– I put tags on each form to be able to identify if the problem came from a particular form but I met functional and non functional cases on the same form.
– the “update existing subscribers?” option didn’t change anything (I already had successes and failures whether the option was activated or not)
– the failures to register on both lists do not seem to correspond to a particular period or to particular changes
– the users registered only on one list were not already registered on the other (tests were done with new addresses)
– there is no other source of newsletters outside the site

Mailchimp support was contacted but we were redirected to you (the developers of the plugin)

I’m looking for any new path to explore. The problem seems to me random (even if it is surprising) and not 100% reproducible for the moment, I unfortunately have no other clues to give you

I hope you will be able to help me,

Regards,
Margot