i have 2 office, 2 different provider and ppp type connection - pptp and pppoe.
router1 (rb3011 //pptp //6.40.55) not create automatic rule in mangle, but i ticked “change tcp-mss = yes” in PPP profile (1 years ago - that rule worked, but router was rb451)
router2 (hex lite //pppeo //winbox 6.34.2) create correct rule in mangle - 1 for in-interface and 1 for outinterface
if i`m not create rules in mangle - internet not work correctly 
ok, maybe your PPPoE client is configured with the wrong MTU.
if you use an ethernet port with MTU set to 1500, your PPPoE client shall have 1492 byte MTU. (PPPoE + PPP header together are 8 bytes)
/interface PPPoE-client set 0 mtu=1492
this way routerOS will be able to figure out the MSS size automatically and adjust it as needed.