Fermentation stops after a few hours, then resumes after half a day

Question

Fermentation stops a few hours after pitching, and seems dormant for half a day, before resuming activity. This only happens in one out of two fermentation buckets, both pitched the same WLP400 (Wit yeast) from the same starter, and residing in the same temperature-controlled fermentation kloset. The monitoring equipment (a couple of PLAATOs) has been tested, and presumably works as expected (i.e. is registering activity when emulated). What is going on here?

Edit: Filling the wort and pouring the yeast was alternated between the buckets, to ensure roughly similar conditions.

Figure 1: Bucket where fermentation seems dormant. Fermentation activity shown by the green line, temperature control shown by the white.

Figure 2: Bucket with expected fermentation activity. Fermentation activity shown by the green line, temperature control shown by the white.

Answer 1

If I had to guess the stalled fermentor was the last to be filled and got less of the wort nutrients.

When splitting a single wort into two fermentations. It's important to alternate the filling of the vessels about 10-20% fill before alternating. This blends the wort, so each vessel gets a share of the nutrients that can drop in the cold break.

Edit: Possibly more oxygen than the other fermentor causing a longer growth / lag phase.