Rocket M5 connection problem

[gabate]

hello,
I have the following scenario:
Site A:
1 pppoe server configured on a routerboard mikotik with wlan1 to site B
Site B: routerboard mikrotik receiving on wlan1 from Site A and wlan2 trasmitting to Site C.
Site C:
1 routerboard mikrotik with eth2: 172.19.1.225/29 it receives on wlan1 to site B
1 Ubiquiti M5 with Rocket ip: 172.19.1.227/29 (connected on eth2 of mikrotik RB), which trasmitting to site D.
Site D:
1 Ubiquiti M5 with Rocket ip: 172.19.1.228/29 it receives from the site is linked to 1 with routerboard mikrotik eth2: 172.19.1.226/29; eth3: 172.19.1.193/29 connected to 1 Ubiquiti M5 with Rocket ip: 172.19.1.196/29 transmitting to site E.
Site E
1 Ubiquiti M5 with Rocket ip: 172.19.1.195/29 it receives from site D connected to 1 routerboard mikrotik with eth2: 172.19.1.194/29; wlan1 connects about 100 PPPoE client.

Problem:
The links between C-D and E-D sites go down every few minutes by dropping the PPPoE clients who register on site A.
If instead I encapsulated the links by eoip-tunnel between the 2 RB mikotik of 3 sites C-D-E the problem does not occur.

Some of you know give me an explanation?
Thanks
Giuseppe