BT performance with small packets

Hi

I have two RB5009UG+S+ with RouterOS 7.4.
Devices are dirrectly connected. Without additionl connfiguration. Only IP addresse on the used ports.
The BT test is started from one router to another. The CLI coomand looks like:

/tool/bandwidth-test 192.168.1.204 duration=60s direction=transmit interval=1 local-udp-tx-size=52 remote-udp-tx-size=52 user=admin password=admin

And the result of the tx-current speed gradually decrease.
From 142Mps at hte start of the test to 9.6Mbps after 30 seconds
The same behavior can be seen with RouterOS v7.5beta5
The same behaviour can’t be seen with two RB4011iGS+

Is there a know restriction of the RB5009UG+S+ or RouterOS 7.X for BT with small pckets?

Thanks in advance