It is difficult to tell the exact reason. It might turn out to be the device in the end, but something specific to your unit only, as I have not been able to replicate problems with band 28.
Such discrepancies as low download/high upload might point to physical layer issues, such as cables, connectors. Could you, for example, perform a speed test, switch port 0 and port 3 antennas, and try speed test again, to check, if the cable does not have any impact on the performance? What are cable lengths between antennas and the router? What is the cable type?
Also, since band 28 is an FDD band, it has separate frequency channels for uplink and downlink streams. In general, far more data is transmitted from base tower to users equipment than the other way around and if you are roaming around edges of tower coverage zones, is possible that interference becomes stronger the further you are from the tower you are connected to.
Low download may also be an indication of high tower load.
It would be interesting to compare another device's reception on band 28 at the same spots.