MP4 export audio sync issue

I have been on Resolve since v17. Pretty techy guy, audio engineer for 30+ years, photo and video for more than 15. Last night I exported a 1080p vertical video in MP4 format (as I usually do), and all of my cuts played back late compared to the audio, in the render. I have never had this happen, in hundreds of exports of MP4's.

I searched for every answer, google, chatgpt, etc. Tried every solutions, nothing fixes it. Frame rates and audio sample rates match, nothing else crazy, it's a 20 second social media commercial for a client.

If I export as a .mov file (which I never use), or ProRes etc, it plays back perfectly in sync. Some sources say Resolve has an issue exporting MP$'s, but I've never seen it until now. I'd REALLY REALLY prefer not to have to render in a non-use format and convert in Handbrake every time this happens?

Anyone have any ideas? Experience similar? All help greatly appreciated!!!

In the MP4 export options "Advanced Settings" there is an option "Enable compatibility with legacy players". Enabling this should fix sync issues on native Windows 10 media players.

Windows 11 doesn't have this problem, and playback in third party media players like MPC-HC is normally fine.

Thank you so much!

In the MP4 export options "Advanced Settings" there is an option "Enable compatibility with legacy players". Enabling this should fix sync issues on native Windows 10 media players.

That's a great tip -- thanks for the answer, Alan.