I have a 2016 MacBook Pro, an iPad Air 3rd generation and an iPhone SE 2020. The MacBook Pro doesn't display any iCloud tabs, while the iPad and iPhone both display all open tabs from both the other devices.
I've tried signing out of iCloud on all devices and on just the Mac and signing back in again, and just toggling Safari off and on in the MacBook Pro's iCloud settings. I did the latter three or four times and it did temporarily fix the problem once, but only for about 24 hours. Also, it may be relevant that to switch Safari back on, I have to click the tick the Safari box and agree to merging data twice instead of once.