Neighbor building in MPLS

imomoe

My topology is like this, and then the configuration is like this. Why can r1r2r3 only have one LDP neighbor? Generally, it is the LDP neighbor between R1 and R2. R3show MPLS LDP neighbor is empty. If I shut down the ports connected between R2 and R1, R2 and R3 can have LDP neighbor. But when I open the ports connected between R2 and R1, the LDP neighbors of R2 and R1 can‘t get up. I‘ve also changed the mirror image. I‘ve tried the simulator GNS3 and eve-ng, and even changed the computer. This is the case. I want to ask why