This bug occurs after using an elytra for any period of time; after using the elytra your avatar will phase through any blocks above their waist, this includes: full blocks, half slabs, stairs and possibly a few more, however I have not experienced them myself yet. Additionally this bug seems to only occur in worlds that have been made and played in survival mode and they only method I have found to “fix” the issue, is to leave and reload your world until the bug then occurs again.