As usual, the answer is alcohol.
This DVD burned fine, but didn’t read. I pressed my nail into the side to separate the 2 disc sections, then played around with it*, and finally stripped both the purple dye and reflective layer with isopropyl alcohol.

*I of course tried what it would do if re-inserted into the DVD drive. Single half wouldn’t spin up. The spindle didn’t have good enough grip. Placing the half with reflective layer back on top surprisingly made it read as a blank DVD, showing its (past) properties.
Trying to burn it again, unsurprisingly, resulted in I/O error.