Experiencing the exact same issue regarding the planecaster.

In my run MC went down during the initial fight with the Inquisitor. I've seen multiple people mention this as the cause for the conversation after not triggering. But no amount of reloading to earlier saves seems to fix the issue and start the conversation.

I've tried:
- giving him the Artifact
- denying him the artifact and killing him
- killing him without talking to him
- making sure I talked to him as MC with artifact in inventory
- killing him last during the fight
- being close to the planecaster with the whole party as we kil him/give him the artifact.

Yet no luck. Also tried getting Lathanders blood first and then coming back to kill them. Didn't help.

I don't have the time to start a full run again and would really like to finish this questline before moving on. Lae'zel is threating to leave the party if I don't complete her thing before continuing the main quest.