A new Mozilla bug report suspects an issue with the load-after-write operation on certain Samsung phones. The problem is due to the ARM instruction LDRB causing a sign extension in w12, leading to reading a value from an incorrect memory page. Check out from the report:

“Since we just stored 0xba, that's what x12 should contain. However, that's not what we're seeing. Looking at a dump, I see x12 = 0x00000000ffffffba. That's not zero-extended.”

This means that the upper bits are not getting cleared. Likely a bug in the Mongoose M5 core used in Non-US Samsung devices.

This is unsurprising, as ARM lacks the verification ecosystem RISC-V has.

In one or two generations, Samsung will have moved to RISC-V, alongside the rest of the industry