MTU changed after switch on internet sharing on MacBook Pro

I checked MTU on my MacBook Pro and found that after switch on Internet Sharing the MTU changed. I don't know if this is an issue or it's normal.

Test env:
macOS Catalina
Version 10.15.7

With thunderbolt ethernet connected run following ping to detect the MTU:

Code Block 
% ping -D -s 8164 1.1.1.1
PING 1.1.1.1 (1.1.1.1): 8164 data bytes
ping: sendto: Message too long
ping: sendto: Message too long
Request timeout for icmp_seq 0
ping: sendto: Message too long
Request timeout for icmp_seq 1
^C
--- 1.1.1.1 ping statistics ---
3 packets transmitted, 0 packets received, 100.0% packet loss

Code Block 
% ping -D -s 1473 1.1.1.1
PING 1.1.1.1 (1.1.1.1): 1473 data bytes
ping: sendto: Message too long
ping: sendto: Message too long
Request timeout for icmp_seq 0
ping: sendto: Message too long
Request timeout for icmp_seq 1
^C
--- 1.1.1.1 ping statistics ---
3 packets transmitted, 0 packets received, 100.0% packet loss

Code Block 
% ping -D -s 1472 1.1.1.1
PING 1.1.1.1 (1.1.1.1): 1472 data bytes
1480 bytes from 1.1.1.1: icmp_seq=0 ttl=59 time=0.987 ms
1480 bytes from 1.1.1.1: icmp_seq=1 ttl=59 time=1.253 ms
1480 bytes from 1.1.1.1: icmp_seq=2 ttl=59 time=1.172 ms
^C
--- 1.1.1.1 ping statistics ---
3 packets transmitted, 3 packets received, 0.0% packet loss
round-trip min/avg/max/stddev = 0.987/1.137/1.253/0.111 ms

The MTU should be 1500.
Then switch on Internet Sharing and ping again:

Code Block 
% ping -D -s 8164 1.1.1.1
PING 1.1.1.1 (1.1.1.1): 8164 data bytes
8172 bytes from 1.1.1.1: icmp_seq=0 ttl=59 time=1.417 ms
8172 bytes from 1.1.1.1: icmp_seq=1 ttl=59 time=1.541 ms
8172 bytes from 1.1.1.1: icmp_seq=2 ttl=59 time=1.508 ms
^C
--- 1.1.1.1 ping statistics ---
3 packets transmitted, 3 packets received, 0.0% packet loss
round-trip min/avg/max/stddev = 1.417/1.489/1.541/0.052 ms

Code Block 
% ping -D -s 8165 1.1.1.1
PING 1.1.1.1 (1.1.1.1): 8165 data bytes
ping: sendto: Message too long
ping: sendto: Message too long
Request timeout for icmp_seq 0
ping: sendto: Message too long
Request timeout for icmp_seq 1
ping: sendto: Message too long
Request timeout for icmp_seq 2
^C
--- 1.1.1.1 ping statistics ---
4 packets transmitted, 0 packets received, 100.0% packet loss

The MTU seems changed to 8192.

With ifconfig it shows

Code Block 
en5: flags=8863<UP,BROADCAST,SMART,RUNNING,SIMPLEX,MULTICAST> mtu 1500

From packet capture could see that what sent out from en5 are Fragmented IP protocol (proto=ICMP 1... and the length is 1514. So although the ping command set don't fragment the packets are still fragmented.

Can someone tell me if this is normal and how it works? Thanks a lot.