I'm currently seeing what appears to be similar on a current (feb/mar 24) snapshot build
https://forum.openwrt.org/t/multiple-bsses-specified-in-bss-config/191177
Did this issue just go away or was there an update or change made?
[edit] Just tried the mod by @DDK and it appears to have resolved the issue after restart with single bss and then adding and removing extra bss but still failed after reboot with multiple bss enabled.