Nice find!
…but would not be a huge problem for SF12, where everything takes much, much longer anyhow, so despite starting a tad too late the LoRa chip still was able to detect the preamble it was looking for when waiting for a downlink. (It doesn’t need the full preamble; this fact is also used for some single-channel gateways that support multiple spreading factors; see Switch bandwidth and SF of node while gateway doesn't? - #5 by arjanvanb)