Telzaro is invisble, cannot be hit.

Telzaro in the Temple barracks doesn't appear except as a red swirl on the ground, cannot be hit, but also doesn't do damage.

Happened to me too today, the boss remains underground and cannot be hit, If you teleport to the checkpoint you can get out of the room and do at least other things.

This happened to me today. Respawn at checkpoint puts you back in the beginning of the temple but he's still bugged underground and cannot be damaged.

This happened to me as well. In my case it was Zoltec, High Priest.
The Boss disappears at ~75% health and doesn't come back. You only see a geyser in his place. You can respawn at the nearest checkpoint but the doors in the room he is in will close as soon as you enter the room.
The only solution right now seems to be to respawn at the nearest checkpoint and avoid the room if you can.
It happened to me in two different temple instances. In the first case he blocked a single room but in the second case he blocked 80% of my temple.
This is especially annoing if you have limb modifications - they will be discarded if you respawn at the checkpoint