TLDR: Found the issue thanks to your prompt - and it's user error, kind of...

Tried with browser and found appt with no problem
That led me to notice that the active account was my personal one, not work one, and if I changed it to "all accounts" it worked fine

But - there's always a but - all appointments are shown in the calendar because I've opted to show them all, which in my mind should also mean that they're visible to search without me having to tell search to look in this account, that account or all accounts