Recently updated contacts API call failing


We’ve been having problems with the recently updated contacts api endpoint (contacts/v1/lists/recently_updated/contacts/recent). Our app requires a client to specify specific contacts they would like us to pull into our system. We do that by having them add contacts to a list in HubSpot that we have mutually agreed upon as being one we will fetch contacts (and any updates on these contacts) from. We use the recently updated endpoint to get the stream of all updated contacts and then filter out the contacts we care about by the list ids.

Recently we’ve had troubles with leads being added to lists but not showing up in the recently updated contacts api endpoint. It appears that adding leads to a list may not update the last modified date on them.
We looked into pulling all leads on the lists we care about but this means we will constantly fetch information about all contacts on the lists which is a lot of overhead in api calls to fetch updates and new leads added to lists.

Is there a way to have these appear in the updated contacts API calls, or suggestions for other methods we could use to pull these contacts?