When gliding with the elytra into any body of water, the Depth Strider 3 enchantment doesn't modify the swimming speed of the player, making them instead swim at the default (id est not affected by any enchantment or effect) speed. Approaching the water while walking or running instead applies the enchantment correctly when swimming, so the issue revolves around the gliding mode specifically.

To reproduce:

1. Equip elytra and a pair of boots with Depth Strider 3 enchantment;

2. glide into a body of water and start swimming (pressing W + the sprint key);

3. Repeat step 1 and 2 without equipping the elytra, walking or running instead into the water and swimming as above;

4. compare the swimming speed difference between point 2 and 3.

The swimming speeds between point 2 and 3 are different, which would be a bug because, as the player swims in the water, Depth Strider 3 should be applied regardless of how they got into it.