Firmware Version: RouterOS 7.1.
Router: HAp ac3 and HAp Lite.
Configuration: 2 Different Tx Chains are configured by enabling the check box.

Scenario Description:
During Association, the non-AP station sent HT Capabilities with the Tx STBC Bit cleared, then the AP Station replies with Tx STBC Bit Set with a status code success, (Then Capabilities Matched and accepted that STBC Is not supported).

During the connection throughput dropped to the half, after captures, it seems like the AP is sending frames with the STBC Feature, where a duplication of the frame is sent on two spacial streams.

First: upgrade to a recent ROS version, latest stable is 7.6

Then: which wireless driver are you using on hAP ac3? Legacy (part of basic ROS package) or wifiwave2 (add-on package)? There are some known incompatibilities between legacy and new driver and you probably should run legacy driver on hAP ac3 in your particular use case (even though this means lower performance).
Which device is used in which mode (AP vs. station)?

wifiwave2 is installed on the hap ac3

First: upgrade to a recent ROS version, latest stable is 7.6

Then: which wireless driver are you using on hAP ac3? Legacy (part of basic ROS package) or wifiwave2 (add-on package)? There are some known incompatibilities between legacy and new driver and you probably should run legacy driver on hAP ac3 in your particular use case (even though this means lower performance).
Which device is used in which mode (AP vs. station)?

wifiwave2

Definitely go up to 7.6. Version 7.1 was the first which had decently working wifiwave2, but it received quite some attention since.