I had tested this a while ago and I believe its expected in case of L2TP/LNS.
The base MTU for si-x/y/z is always 9192 & inet mtu in this case would be 1500.
re0# run show interfaces si-1/0/0.0
Logical interface si-1/0/0.0 (Index 321) (SNMP ifIndex 590)
Flags: Up Point-To-Point SNMP-Traps Encapsulation: Adaptive-Services
Input packets : 0
Output packets: 0
Protocol inet, MTU: 9192
Max nh cache: 0, New hold nh limit: 0, Curr nh cnt: 0, Curr new hold cnt: 0, NH drop cnt: 0
Flags: Sendbcast-pkt-to-re, Receive-options, Receive-TTL-Exceeded
re0# run show subscribers
Interface IP Address/VLAN ID User Name LS:RI
si-1/0/0.3221226232 10.xx.0.74 karand@l2tp.net default:default
re0# run show subscribers extensive
Type: L2TP
User Name: karand@l2tp.net
IP Address: 10.xx.0.74
IP Netmask: 255.255.255.255
Domain name server inet: 10.x.x.3 10.x.x.3
Logical System: default
Routing Instance: default
Interface: si-1/0/0.3221226232
Interface type: Dynamic
Underlying Interface: si-1/0/0.3221226232
Dynamic Profile Name: DYNAMIC-PROFILE-1
Dynamic Profile Version: 1
State: Active
Radius Accounting ID: 12223
Session ID: 12223
PFE Flow ID: 848
Login Time: 2018-08-23 14:05:46 IST
IP Address Pool: dhcpv4
re0# run show interfaces si-1/0/0.3221226232 extensive
Logical interface si-1/0/0.3221226232 (Index 536871760) (SNMP ifIndex 200000848) (Generation 808)
Flags: Encapsulation: Unspecified
Bandwidth: 0
Traffic statistics:
Input bytes : 1104
Output bytes : 192
Input packets: 24
Output packets: 24
Transit statistics:
Input bytes : 1104 0 bps
Output bytes : 192 0 bps
Input packets: 24 0 pps
Output packets: 24 0 pps
Protocol inet, MTU: 1500 <<<<<<<<<<<
Max nh cache: 0, New hold nh limit: 0, Curr nh cnt: 0, Curr new hold cnt: 0, NH drop cnt: 0
Generation: 0, Route table: 0
Flags: Unnumbered
Donor interface: lo0.0 (Index 324)
Addresses, Flags: Is-Primary
Destination: Unspecified, Local: 10.xx.0.1, Broadcast: Unspecified, Generation: 0
re0# run show ppp interface si-1/0/0.3221226232 extensive
Session si-1/0/0.3221226232, Type: PPP, Phase: Network
Keepalive settings: Interval 30 seconds, Up-count 1, Down-count 3
RE Keepalive statistics:
LCP echo req Tx : 0 (last sent: never)
LCP echo req Rx : 0 (last seen: never)
LCP echo rep Tx : 0
LCP echo rep Rx : 0
LCP echo req timeout : 0
LCP Rx echo req Magic Num Failures: 0
LCP Rx echo rep Magic Num Failures: 0
LCP
State: Opened
Last started: 2018-08-23 14:05:46 IST
Last completed: 2018-08-23 14:05:46 IST
Negotiated options:
Authentication protocol: pap, Magic number: 1581768641, Initial Advertised MRU: 1500
Authentication: PAP
State: Grant
Last started: 2018-08-23 14:05:46 IST
Last completed: 2018-08-23 14:05:46 IST
IPCP
State: Opened
Last started: 2018-08-23 14:05:47 IST
Last completed: 2018-08-23 14:05:47 IST
Negotiated options:
Local address: 10.xx.0.1, Remote address: 10.xx.0.74
Negotiation mode: Passive
re0# run ping 10.xx.0.74 size 1432 count 1
PING 10.xx.0.74 (10.xx.0.74): 1432 data bytes
1440 bytes from 10.xx.0.74: icmp_seq=0 ttl=64 time=1.348 ms
--- 10.xx.0.74 ping statistics ---
1 packets transmitted, 1 packets received, 0% packet loss
round-trip min/avg/max/stddev = 1.348/1.348/1.348/0.000 ms
This packet consist of: IP + UDP + L2TP + PPP + IP + ICMP + DATA.
Doesn't change in case if L2TP (and i'd try to return framed-mtu xxxx mtu from AAA)
BTW, for tcp-mss, it aids to limit/aviod or say prevent packet fragmentation and to protect against packet loss that can occur when a packet must be fragmented to meet the MTU size.
More info here:
https://www.juniper.net/documentation/en_US/junos/topics/concept/pppoe-subscriber-access-mru-mtu-overview.html