Can a failed write to Read/Write partation on SD card cause corruption of a Read Only partition?
So, are partitions on an SD card treated independently at the card level…?


No.

The partition table on an SD card is just a specific piece of data written to the first sector. It doesn't have any implications for the hardware, and most flash controllers on SD cards will ignore it.

There are essentially no bounds on what a buggy flash controller can do if the card is powered down while a write is in progress -- in a worst-case scenario, internal flash translation data could be corrupted, "bricking" the card. Even in less dramatic scenarios, data can potentially be corrupted in any location on the card.

In practical terms, keeping your read-only data isolated to a separate partition will usually be safer than keeping it on the same partition. But there are no guarantees, especially if you're using cheap cards.