After installing DNL, the changed Ender Dragon from True Ending dies the moment it gets summoned, naturally and via command.

There's nothing I changed about Ender Dragon so I'm not sure if it's from DNL.

Unfortunately I am also experiencing this issue. I tried going into the end without DNL and with True Ending and the dragon didn't instantly die but with both true ending and DNL installed the dragon did instantly die, regardless of other end mods installed.

I see, I'll try to fix it. Thank you for the report

It seems like the developer of the true end doesn't have the source code public...