Primary interface address is not always used when VIPs are present
If you have IP Aliases on a WAN interface that a Site to Site IPSec tunnel is riding over and upgrade from 2.4.5p1 to pfSense Plus, you have to go into the WAN interface and hit "Save" and "Apply Configuration" then restart the IPsec service to bring tunnels up post-upgrade. Otherwise IPSec will never connect no matter how many times you cycle the service.
Step by Step:
1. Create IPSec on WAN interface with several IP Aliases
2. Upgrade to 21.02/21.02p1
3. IPSec is broken, so you go into the WAN interface, hit save with no changes, and Apply Changes.
4. Restart IPSec service
Tunnels now work.