I have around 25 rules that move mail into folders. Last week they stopped running automatically. If I click Run Rules Now they work. Exchange account in Outlook 2019. Why?

Two common causes.

Rule size limit: on Exchange, all your rules together must fit in a quota, 256 KB by default. Long rules with many senders or words eat it fast. If you are near the limit new rules are disabled with a message, and older ones can stop. Merge rules, delete old ones and shorten long lists of names.

Rule errors: open File > Manage Rules & Alerts and look for a rule with a red mark or a rule that points to a folder that was renamed or deleted. Fix or remove it.

Also check that Outlook is not in a state where the rule is client only. Client only rules run just when Outlook is open. A rule that mentions a specific file or a sound is one of those.

I had one rule with a huge list of senders. Split it and removed old rules. Everything runs again.