Summary
The Elder Guardian hitbox intersects with blocks when they spawn, causing them to be able to travel through solid or glass blocks, and the world border.
Expected:
Elder Guardions are pushed away from the blocks they are slightly intersecting on spawn, instead being allow to jump on them (like players who intersect a door).
Current behavior:
Elder Guardians appear to choose a random direction to jump (as if they are on land, or rather, not in water), and jump towards that direction. If they are inside a block or the world border, they pass through that block without issue.
Moesh
I've done some thorough and further investigating regarding this issue and wish to shed some light on a couple of things. 🙂
This issue can be reproduced using both the "/summon" command and with spawn eggs, but where this begins to become a problem, however, is with spawn eggs. If you were to spawn an elder guardian close to some blocks (using spawn eggs), you'd notice that they are sometimes able to pass through them due to their hitboxes intersecting blocks upon being summoned, therefore leading me to believe that this ticket duplicates MC-103934.
This core issue as a whole doesn't only affect elder guardians. It affects any entity with wide hitboxes.