This changed in Numbers 13.2. Happens on multiple machines of mine. Tabbing no longer tabs out of cells, it adds tabs in the formula instead (which is worthless to me). When I restored 13.1 from Time Machine backup, the original and intended functionality is restored, which means it's not third-party software or configuration settings. This is not mentioned in the patch notes for Numbers 13.2. I recommend restoring from backup and submitting feedback to Apple Support. I've downgraded and currently am ignoring updates until the correct behavior is restored.