I have this problem every single time I do an OS upgrade or restore my Mac or iPhone. I have been experiencing this exact issue on the last few revisions of Mac OS and iOS. For whatever reason, text replacements syncing is EXTREMELY FINICKY and super frustrating to deal with. These are the steps that have worked for me, but ymmv.
These are the steps I take when I notice they stop syncing - you WILL lose any current text replacements so write them down or save them to a text file or screenshot or whatever.
Also, before you start you should copy the contents of your iCloud Drive to a flash drive or external hard drive, just in case. I don't have a lot of files stored in my iCloud Drive so it's not a big deal for me to pull them down again.
When I perform these steps I like to plug my MacBook into my router with an Ethernet cable to ensure there are no connectivity issues.
1) On your Mac, go to System Preferences > iCloud > Uncheck iCloud Drive. Select Remove From Mac and WAIT for it to finish syncing / updating.
2) WAIT a few minutes. Seriously, just let the computer sit there.
3) Close all open windows and RESTART (Don't shut down, restart. There's a difference) the Mac.
4) Log on and WAIT for a few more minutes, just for good measure.
5) While waiting, turn off iCloud Drive on your iOS devices in Settings > iCloud > iCloud Drive. Wait for it to finish syncing / updating.
6) Turn off your iOS devices and WAIT for a few minutes before rebooting.
7) On your Mac, when you go to the text replacements in the keyboard preferences there should only be one, the default one that says "On my way!"
8) Power on your iOS devices and in the text replacements there should only be one, the default one that says "On my way!"
9) Turn iCloud Drive back on, starting with the Mac. WAIT a few minutes and then REBOOT again.
10) Do step 9 on the rest of your iOS devices. Be sure to WAIT then REBOOT.
Once all is said and done, your text replacements should now sync between all your devices.
Signing out then signing back into iCloud altogether with 2FA is a major pain in the *** (more than performing the steps above) and doesn't always resolve the text replacements syncing issue for me.