RIP changes route after a half day

Hi all,

I have a Problem with the routing protocoll RIPv2.

I have 3 VLANs:

VLAN500 (lan4) for all internal VLANs which should have a interVLAN connection through the Network to other Fortigates.

VLAN501/VLAN502 (lan4) which role is defined as WAN. These VLANS comes from other Fortigates (say B) and provide an internet access.

Actualy the Fortigate (A) has his own modem which should dissapear in a near future. The WAN Port and VLAN501/502 are in a SD-WAN.

These fortigates are connected to each others with Cisco SG350-XG switches.

All devices have RIPv2 activ. All works fine.

internal IP-Ranges FG A for VLAN500:  10.154.x.x, VLANs 100-199

internal IP-Ranges FG B for VLAN500:  10.155.x.x, VLANs 200-299

internal IP-Ranges FG C for VLAN500:  10.156.x.x, VLANs 300-399

Problem:

After a day the FG (A) changes his routes from VLAN500 to VLAN501. Therefore are my policyrules negoiable.

Why does it changes the routes?

Are there any Logs which shows why he changes the route?

Best Regards

Roger

Pictures:

Interface FG A:

Interface FG B:

Staticroutes FG A:

RIP (This settings on FG A/B are the same):

Policy FG A:

Routing Monitor FG A: