How likely is it that I've encountered a MemTest+ bug?

I would say it is almost certain that you have...

see this answer Memtest freezing at...

I can confirm that I run into this issue with virtually every machine I test. Lately I have been working with refurbished lenovos of the T5xx range, with various memory and CPU configurations (usually the stock RAM the machine was sold with). I'm up to at least 10 where I've tried the SMP mode -- usually I just let the failsafe mode run 'cause I need to move along.

My hangs seem to occur at varying tests, but the machines always hang. And hang completely - so the keyboard is unresponsive, requiring a power off.

I have just experienced it with a new Zotac SFF PC, with a 4-core Celeron N3150 @1.6 GHz (stock) and 8 GB of Corsair valueselect DDR3L modules (2 modules) CMSO4GX3M1C1600C11

This machine hangs at Test #7 [Block move], same region both times.

The "Fail-safe" mode runs through this test with no problems, and completes with the memory all testing OK.

Interestingly (maybe to the memtest+ devs) the % completion (progress reporting) is different for the SMP and "fail-safe" modes at the point where the SMP run fails.

In this case, the memtest86+ is the one installed with non-uefi xubuntu 16.04.

Any news from the memtest+ developer?