I have 2 networks connected via l2tp.
Connection working properly. This is server side.

6 ADC 172.16.0.0/24 172.16.0.1 bridge 0
7 A S 192.168.88.0/24 <l2tp-kam> 1
8 ADC 192.168.100.100/32 192.168.100.20 <l2tp-kam> 0

192.168.88.0/24 is network from remote network.
But l2tp-kam is dynamic so if client-server lost connection and then succusfully reconnect there are these routes.

6 ADC 172.16.0.0/24 172.16.0.1 bridge 0
7 S 192.168.88.0/24 *F0076C 1
8 ADC 192.168.100.100/32 192.168.100.20 <l2tp-kam> 0

192.168.88.0/24 has uknown gateway because l2tp was disconnect and interface l2tp-cam is missing.
After reconnection this route staying with uknown gateway.
I have to fix it with add route via l2tp-cam manually.
If I used l2tp server with ROS7 there is no such problem, this issue is on the ROS6 inly.

How can I fix it?
I`ve thought I have to make bridge (e.g cambridge) and I have to add interface list (e.g. CAM) a this inteface list has interface l2tp-cam so then I tried to add route 192.168.88.0/24 via cambridge....but I `won`t able to ping any Ip from 192.168.88.0/24.

Make a static binding in /interface/l2tp-server menu

and there is one more tip , use in profile -> only-one=yes

Sorry guys I didn`t know what you tried to tell me becasue I can`t see any binding in l2tp server menu or only-one=yes in profiles.
But I`ve read https://wiki.mikrotik.com/wiki/Manual:Interface/L2TP and I ty to add route to cam secrets directly.
And it`s workig.
This si rouiing table after reconnecting.

7 S 192.168.88.0/24 *F00002 1
8 ADS 192.168.88.0/24 <l2tp-kam> 1
9 ADC 192.168.100.100/32 192.168.100.20 <l2tp-kam> 0

No. 7 is unknown relic from previous connection, but new connection add route no. 8 automatically.

Edit: This is final settings and there is no more "ghost" route in print.

3 name="kam" service=l2tp caller-id="" password="" profile=default
local-address=192.168.100.99 remote-address=192.168.100.100
routes="192.168.88.0/24 192.168.100.100" ipv6-routes="" limit-bytes-in=0
limit-bytes-out=0 last-logged-out=jun/10/2024 10:47:59
last-caller-id="" last-disconnect-reason=nas-request

7 ADS 192.168.88.0/24 192.168.100.100 1
8 ADC 192.168.100.100/32 192.168.100.99 <l2tp-kam> 0

ppp-->click on the plus and select binding (l2tp in your case). That is for the server site (not sure how to type that in terminal , probably @mrz can help).

From client site where is creating dynamic interface (potential issues in v6, if you play with vrf) profile -> only-one=yes