When running a trident killer, spiders are frequently shunted outside of the solid blocks surrounding the kill the chamber. This allows them to roam freely and kill the player. Also, skeletons are able to fire arrows and attack the player through the solid walls of the trident killer.

To reproduce...

For the skeleton part:

Seed is (negative)4212966053895285705. Ancient City is roughly at (711, -42, 43). There's a mineshaft at one end of the Ancient City. If you're at the center of the Ancient City and facing the mineshaft, the skeleton spawner is on the left edge of the Ancient City, slightly higher than the Ancient City's structures.

Build a trident killer that is fed by the Skeleton spawner. Build a spawnproof/mobproof room adjacent to the kill chamber. AFK until you're attacked. This could take up to 8 hours. I've only AFK'ed there overnight and awoken to being killed.

For the spider part:

Build a hostile mob spawner high enough that it's outside of the Ancient City/Deep Dark biome's boundaries. Have the drop chute drop down to the aforementioned Ancient City's coordinates. I used Powder Snow in the drop chute to prevent falling damage from killing the falling mobs.

AFK until spiders start getting shunted outside of the trident killer's kill chamber. This usually doesn't take more than a few minutes if you're standing next to the bottom of the kill chamber.

This is just one example. I've experienced the same issue in other worlds with different seeds and at different elevations.