The bug

When the player changes their movement state between standing, swimming, sneaking, and gliding, the position regarded as their eye height does not change. This hinders the use of the /execute anchored command, as the purpose is to account for dynamically changing eye positions.

Steps to reproduce:

• Create a new world with the Upcoming Creator Features experiment enabled (or use an existing world).
• Place down a repeating command block, enable the Always Active setting, and insert the following command in it. This should place a particle in front of the player's eyes using the new /execute anchored.

  /execute as @p at @s anchored eyes run particle minecraft:basic_flame_particle ^ ^ ^3
  

• Change your movement state, such as sneaking or swimming.

→ The particle's offset remains the same, at about 1.5 blocks up, instead of adjusting for your new eye level.