I'm wondering if selecting the "View" button, followed by "Show All Devices" might shed some light here?
Usually when I have trouble partitioning and it involves the drive I'm booted from, I will start up from Internet Recovery Mode or (preferably, for loading times) a version of macOS I've previously installed on an external hard drive or USB. Regular Recovery Mode might be fine as well, but it's still booted from the hard drive you're working on.
Internet Recovery can be started by holding the Option, Command and R keys while the computer turns on (you can let go once a globe icon or Apple logo appears).
Before you're starting up to do this, make sure you have a current backup!!! :)
One other thing I can think of that would stop you doing this is if macOS had to move the Recovery HD partition to be between your Macintosh HD partition and your Boot Camp Partition. It doesn't always have to do that, but if it does, you might have to manually delete the Recovery HD partition via Terminal and then expand Macintosh HD.
I'd suggest you use Apple's "Recovery Disk Assistant" to make a Recovery HD on an USB or external Hard Drive before doing that, in case you need to reinstall later, cause I don't think you can put Recovery HD back on the internal drive without a Major OS upgrade (like 10.12 > 10.13) or a reinstall.
If you run:
diskutil list disk0
in Terminal, it will give you a listing of all the partitions on the first mounted disk.
If there is a volume labeled "Recovery", and then a unformatted amount after it, that's why you can't expand Macintosh HD, but if it lists empty space and then the "Recovery" after that, then you can probably just expand Macintosh HD with a external hard drive version of macOS or Internet Recovery.
And again, Backup all your files first please!!!!