Ever since the new xbox windows 10 app started rolling out onto windows 10 devices to replace the xbox companion app, any xbox achievements unlocked on Windows Phone 10 will not sync to your xbox live profile. Simply put, achievements will unlock on Windows Phone 10 but won't ever sync them to your xbox live profile as being unlocked. Even if you still keep the old companion app and don't update it still doesn't sync. It really seems like that the release of the new xbox windows 10 app, it broke the way of how Windows Phone 10 achievements are synced with xbox live profiles.
The last recorded sync I've seen is October 29th 2020, since then no one else has been able to sync their achievements with their xbox live profile.
On a side note Windows Phone 7, 8 and 8.1 achievement syncing has been broken since March 2020 and that still hasn't been fixed, information regarding that is here:
This is a service issue that is happening on all WP10 devices and is something that can fixed by fixing your backend.
Everyone is having this issue, I hope Microsoft takes this issue seriously and fixes it soon.
EDIT: This issue may most likely be occurring as Microsoft is updating Xbox Live services on their end, probably in preparation for the Series X/S, and they're not checking if older platforms are still compatible with everything. In this case, the service that runs in the background for syncing Windows Phone 10 Achievements broke/wasn't tested for compatibility when they updated Xbox Live services. Hence, this is a service issue, please fix this.
EDIT March 18th 2021: I can confirm that Windows Phone 7, 8 and 10 Achievement Syncing has been fixed!! It seems that UWP games aren't fixed like Minecraft (WP), nonetheless thank you very much microsoft for caring and listening to the community :D
On behalf of all of us we thank you :)