Issue:
PPPoE rate limiting isn’t working correctly in 6.34.4 and even more so in 6.35RC45

Description:
This is a simple site with the router setup only to route between the backhaul segment and the sector segment for that tower with PPPoE server running on the sector network segment.

We were testing both of these firmware version at a low client load tower site that was running 6.28 normally.

We upgraded to 6.34.4 and added:
/ip firewall filter add action=fasttrack-connection chain=forward comment="defconf: fasttrack" connection-state=established,related
After this the client testing would max out the available capacity during a speed test on the downstream but match the speed plan correctly on the upstream. (85Mbps/4Mbps)

So we upgraded to 6.35RC45:
After this the client testing would max out the available capacity during a speed test for both downstream and upstream. (~85Mbps/~65Mbps)

Then we downgraded to 6.28:
After this the clients speed plan functioned normally with rate limiting. (~8Mbps/~4Mbps)

Versions affected:
6.34.4, 6.35RC45, and possibly more 6.3X versions

How to reproduce:
Follow steps in description

Notes:
[Provide any notes or additions you would like]

Where you have PPPoE limit on client side or server side PPPoE connection ?

/ip firewall filter add action=fasttrack-connection chain=forward comment="defconf: fasttrack" connection-state=established,related

If you read wiki you will see add this on router disable some packet processing like as bandwidth limitation (queen and some other thing) and this is very usefully in client equipment you can pass much more bandwidth on some device sacrifice some of options
If you limit speed on client side it is not very good practice please, on server side where is pppoe server you not need this rule.
I can only recommend to hire some consultants to check you network and give you some advice or need to post more detail configuration. My opinion for you post working like as expected because in old version 6.28 fast track you posted rule not working with pppoe in never version working.

It becaused fasttracked. disabled your fasttracked.

I was aware that not all packets in a connection can be fasttracked, but I didn't realize that such functions could be lost in the attempt to fasttrack other traffic passing through the router. I'll update the firmware, disable fasttrack related rules, reboot, and run the tests again.