tizoc - I had not yet tried the combination of starter with oats and shaking.
until today.
I just got starting materials for my next batch and will get back to you on that in about a week or so. One week later: The outdoor temperature today was above freezing and I got to brew.
Starter without oats v. starter with oat flakes - to compare Krausen levels
100 gms of golden light DME in 1L of RO water was boiled in saucepan 1 for 5 min, then half (500 ml) was transferred to a second saucepan 2. To saucepan 2 was added 5 gms of flaked oats (not quick oats) and contents of both saucepans were boiled for an additional 3 min. Both were allowed to cool to room temperature (19 ºC) before transfer to sanitized plastic soda bottles of 2 L capacity. Saucepan 2 was poured through a sanitized strainer to remove soggy oat flakes. Both plastic bottles were shaken by hand for 45 seconds. The amount of foam in #2 (oats) was slightly less than foam in #1. Then 5.5 gms of US05 dry yeast was added to each plastic bottle and shaken for 5 seconds. After 4 hours, the foam/Krausen in bottle #1 was greater than in bottle #2 (oats). The liquid in bottle #2 was darker than the liquid in #1. After another 4 hours, the volume of Krausen in #1 was about double of that in #2.
Both have since been pitched to the same batch of wort. The difference will no longer be tracked.
One experiment on US05 in 'shaken, not stirred' starter with oat flakes showed less Krausen than without oat flakes - agreeing with your observation.
Does it mean anything?
Next experiments would be to split the batch of wort and separately pitch the two starters. That was not for today, sorry.