This seems to be a cross platform issue and has had troubles since at least 2015, by the look of previous posts.

I have found that a Fortune II enchantment continues to allow a shovel to mine gravel without issue, and your flint drops improve appropriate to the enchantment. However, if the shovel is increased to a Fortune III enchantment, it discontinues gravel drops when mining these blocks. Flint will drop, as per the enchantment's improvement, but it will fail to produce gravel all-together.

This was true for me on both the Bedrock edition and the Xbox One edition. People on Java are apparently seeing this problem too in years past. I've resorted to using a lesser shovel, but it seems like the better shovel should actually be better and not detract from your resource accumulation.

I've seen this issue while using a Diamond Shovel with the following enchantments:

• Fortune III
• Unbreaking III
• Mending I
• Efficiency V