I2C slave register_model example program erratic behavior

[mni-korfhage]

I'm running the Wire register_model example on an ATtiny416 (running on 3.3V with a 10 MHz clock), and reading from a Raspberry Pi using the smbus library, and the line that does the reading is

data = bus.read_i2c_block_data(DEVICE_ADDRESS, 0, NUM_BYTES)

Nothing else is on the I2C bus, which has 4.7K pullups. The clock is 100 KHz.

I am sporadically getting bad data. I've hooked this up to a DSLogic U3Pro32 with a 100 MHz sampling rate, and a good read looks like this:

Frequently, however, it looks like this:

It starts ok, as they all do, but it looks like the ATtiny stops sending data so the master always reads 0xFF. The point at which it goes bad varies. Here's another example where it only sent 2 good bytes before turning bad:

Does anybody recognize this behavior?