Hello,
there are many bugs in Unreal Engine with ARCore and ARKit. Other users have reported similar problems.
These problems are tracked at https://issues.unrealengine.com
If you search for AR or ARCore you can find an issue similar to yours, it is probably the same bug.
(Please note since yesterday, feb.12th, the bug report site is offline, but the link is correct, so you need to try again later.)