I have noticed some of the same problems with my CRS328-24P-4S+RM device, it looks to be related to latency, and maybe packet buffers in the switch chip.
On the forum there are other posts of people having similar issues with other CRS models. It would be nice if mikrotik could respond to this issue.
An iperf3 on a local low latency network from 10G port to 1G port gives around 930Mbit both ways, but if something with higher latency is used like an internet connection it can drop down to almost nothing (30-130Mbit) in the 10G to 1G direction, while 1G to 10G is full speed.
It appears to be some kind of burst problem on the hw level, as the problem can be solved by limiting (queue) the traffic coming into the 10G port, that is destined to the 1G port to below 1G speeds.
First reply post didnt get though for some reason, so I try again, sorry if 2 replies show up.