1)Place a water bucket onto the ground (there MUST be vines present in the placement block) [aka waterlog the vines]
2)pick the water back up with a bucket
The water won't go back into the bucket, and the water source block disappears visually. The flowing water looks normal, and you can pick the water source block up with the bucket, as long as you use the empty bucket on the now “empty” source block
This issue is being tracked at MCPE-218044.