On opt-in in intial user session, the queue is never flushed

[bvanderdrift]

Bug Description

I have an issue with data not showing up on first session only. I debug in live view

When I act as a new user (anonymous browser) the following happens:

How to reproduce

• init with opt_out_capturing_by_default: true
• Cookie banner is shown
• I accept
• $opt_in event is correctly received in live view
• I change page -> console logs [PostHog.js] send "$pageview" but no event shows in live view
• I change page again -> console logs [PostHog.js] send "$pageview" but no event shows in live view
• I change page once more -> console logs [PostHog.js] send "$pageview" but no event shows in live view
• I wait 1 minute (tried with 10 minutes too just to be sure)
• I click a button -> console logs [PostHog.js] send "$autocapture" but no event shows in live view
• at this point no other event than $opt_in are shown in the live events view
• I refresh the page
• now all the events (3x page view and button click) show up backdated correctly.

This is reproducable 100% of the time.

Any future sessions work as expected -> events are correctly periodically flushed from now on

Additional context

posthog-js: 1.269.1

Opt-in logic

export const usePosthogStateSyncEffect = () => {
  const { isAuthenticated, isLoading, user } = useCurrentUser()
  const posthog = usePostHog()
  const didUserAllowTracking = useDidUserAllowTracking()

  useEffect(() => {
    if (isLoading) return

    if (user) {
      posthog.identify(user._id)
    } else {
      posthog.reset()
    }
  }, [isAuthenticated, user, posthog])

  useEffect(() => {
    if (didUserAllowTracking && !posthog.has_opted_in_capturing()) {
      posthog.opt_in_capturing()
    } else if (didUserAllowTracking === false) {
      posthog.opt_out_capturing()
    }
  }, [posthog, didUserAllowTracking])
}

init logic

<PosthogProvider
  options={{
    api_host: config.posthog.apiHost,
    defaults: '2025-05-24',
    opt_out_capturing_by_default: true,
    cookieless_mode: 'on_reject',
    capture_exceptions: true,
    debug: !config.isProduction,
  }}
>
...

Debug info

Session: https://us.posthog.com/project/sTMFPsFhdP1Ssg/replay/0199ab5a-656d-76ec-b952-f4fae2dba896?t=1112
Admin: http://go/adminOrgEU/0199a963-d0ff-0000-8186-3a87be298b7b (project ID 92410)

[pauldambra]

@robbie-c Josh is OOO now but this issue i guess relates to #2353

[pauldambra]

cc @thmsobrmlr for sdk support this week

[thmsobrmlr]

This was fixed in #2408, and will roll out with posthog-js@1.273.0.

[bvanderdrift]

Thanks team, really appreciate the fast resolution 🙏