This is “normal”.
It is a general change in A11 and the option is now reported to be deprecated by Google.
For FPOS the disappearance of this toggle is being treated as a “feature request” to put it back.
For those who can control the router, a work-around is to have the router use different network IDs for the two frequency bands. If you’re interested in discussing this please start a new thread and link back here.