Downlink messages not conforming to LoRaWAN size limitation when send in RX2

to reduce the number of downlink messages, I tried to put as much information as possible in one downlink message. Therefore I tested putting as many bytes into a downlink message as the spreading factor of the SF as a message on that spreading factor can have.
When I tried with SF7 I generated downlink messages with 222 bytes, which is the maximum for this SF. But I noticed that about 4% of the downlink messages were send in RX2 and with SF9. Now the downlink messages were too large for the new SF and thus not conforming to the LoRaWAN specification, but they were send nevertheless.

Is TTN not enforcing the maximum packet sizes of the LoRaWAN specification?
Is there any way to force TTN to use RX1?
Or is there any other way to avoid this issue other than splitting the downlink data in multiple packets that fit into SF9?