Rx2_frequency config being ignored

I have a Class C network using subband 2 (channels 16-23) and RX2 DR 10, channel 923.9MHz. In other words, I have the following lines in chirpstack-network-server.toml:
rx2_dr=10
rx2_frequency=923900000

For the multicast group, I also have DR 10 and 923900000 set.

I have found that ALL class C downlinks (ie not multicasts) are at 923.3MHz instead of 923.9MHz.

Multicasts transmit over 923.9MHz, and class A downlinks also work.

Is this right?