I used Outlook Web Access (OWA), the web version of Outlook, as my only email client at work for the past few years. I had created rules for filtering emails in it. When the COVID work from home began I switched to using the Windows application of Outlook. After a week or so I was informed by my colleagues that I had not been responding to certain important emails.
I looked up the list of rules in Outlook-Win and noticed right away that some of my rules that existed on OWA were not listed. And with those rules missing, other delete-email rules I had at the bottom had been deleting my important emails.
When I opened the rules list in OWA, the missing rules had been moved down to the bottom of the list and were disabled.
It was then I remembered that I had opened the Rules list in Outlook-Win one day to add a rule and at that point it had warned me that some rules were created with another client and might not work. Only now did I realize that the other client it was referring to was OWA! The strangest behavior here is that the offending rules of OWA had been working in Outlook-Win until I opened the Rules list in Outlook-Win. At which point, Outlook-Win disabled them.
I needed those rules, so I recreated them in Outlook-Win. I noticed that Outlook-Win added the suffix (client-only) to those rules. A bit more investigation revealed that there were certain rule actions that were client-only: that is they were applied not by the mail server, but by the email client (Outlook-Win or OWA). And one such rule action is to apply a category on an email. For whatever weird Microsoft reason, this action makes a rule client-only.
That is, if you add a rule like this on OWA, Outlook will not open the rule and will silently disable it. If you add a rule like this in Outlook-Win, it will add a client-only suffix and when you open the rules in OWA, it will disable them.
Since I did not want to deal with this craziness and I did want to keep the flexibility of checking my email both in Outlook-Win and OWA, I created rules without applying category. And my new rules appeared in both Outlook-Win and OWA and worked in both clients.