LTE tx-queue-drop with Queue Tree

Hi,

I see a strange behavior when I attach a queue tree on lte1:

/queue tree
add max-limit=1M parent=lte1 queue=fq-codel-ethernet-default packet-mark=no-mark

The actual queue type does not matter. I tried cake, fq-code and fifo queue types. Always the same.

My LTE upload is always at least 6 Mbit/s. Usually at 9-10 Mbit/s.

With the queue tree enabled, the lte1 interface tx-queue-drop keeps increasing during speedtest.

/interface/reset-counters lte1

Start ookla speedtest on any local device.

/interface/print stats interval=400ms proplist=tx-queue-drop where name=lte1
Flags: R - RUNNING
Columns: TX-QUEUE-DROP
#   TX-QUEUE-DROP
0 R         8 388

Ookla reports 0,85Mbit/s upload. So the queue tree actually is effective.

At the same time queue tree stats show dropped=0.

Without the queue tree, lte1 tx-queue-drop stays at 0. Always.

I also tested bucket-size=0, same result.

Does anyone know or has a theory what exactly causes tx-queue-drop on an LTE interface in this case?

I have a MikroTik support ticket open: SUP-222002. Initially I observed and reported an issue where the tx-queue-drop jumped from 14xx (4 digits) to tx-queue-drop=4 294 943 787 in a single Winbox "repaint". But now I am questioning why there are tx-queue-drop at all. I can't explain/see where the interface buffer could "overflow".