The bug

The area in which the collision of the player and a block is tested while a player is flying seems to be larger than the hitbox of the player.
The attached screenshot shows the moment in which the game claims the player collided with the wall.

How to reproduce

1. Use this compacted command

   /setblock ~ ~1 ~ standing_sign 8 replace {Text1:"{\"text\":\"Click\",\"clickEvent\":{\"action\":\"run_command\",\"value\":\"/summon armor_stand ~ ~10 ~ {NoGravity:1b,Tags:[\\\"elytraMarker\\\"]}\"}}",Text2:"{\"text\":\"\",\"clickEvent\":{\"action\":\"run_command\",\"value\":\"/setblock ~ ~4 ~8 sandstone\"}}",Text3:"{\"text\":\"\",\"clickEvent\":{\"action\":\"run_command\",\"value\":\"/replaceitem entity @p slot.armor.chest elytra\"}}",Text4:"{\"text\":\"\",\"clickEvent\":{\"action\":\"run_command\",\"value\":\"/setblock ~ ~ ~ standing_sign 8 replace {Text1:\\\"{\\\\\\\"text\\\\\\\":\\\\\\\"1. F3 + B\\\\\\\",\\\\\\\"clickEvent\\\\\\\":{\\\\\\\"action\\\\\\\":\\\\\\\"run_command\\\\\\\",\\\\\\\"value\\\\\\\":\\\\\\\"/tp @p @e[type=armor_stand,tag=elytraMarker,c=1]\\\\\\\"}}\\\",Text2:\\\"{\\\\\\\"text\\\\\\\":\\\\\\\"2. F5 (1st)\\\\\\\",\\\\\\\"clickEvent\\\\\\\":{\\\\\\\"action\\\\\\\":\\\\\\\"run_command\\\\\\\",\\\\\\\"value\\\\\\\":\\\\\\\"/tp @p 0 20\\\\\\\"}}\\\",Text3:\\\"[\\\\\\\"3. click again\\\\\\\"]\\\",Text4:\\\"[\\\\\\\"\\\\\\\"]\\\"}\"}}"}
   

2. After about one second press the space bar to start flying