The answer to 'why is my gutter guards missing shingles' is that the gutter guards may be causing damage to the roof shingles. Gutter guards can sometimes be installed improperly or put too much pressure on the shingles, leading to them becoming loose or falling off. This can happen over time as the shingles age and the guards wear them down.
Other common causes include high winds, heavy rain, or ice buildup that can dislodge the gutter guards and damage the shingles. Older, brittle shingles are more prone to this issue. If the problem is widespread, it's best to have a professional roofer inspect the roof and gutter system to determine the root cause and make any necessary repairs.