If the Y and A axes are moving at different speeds, that suggests that something like the "steps per" settings are different, or maybe the driver DIP switch settings are different - but neither explains your other symptoms! I almost suspect ESS hardware problems, but it's very difficult to diagnose remotely like this.

My CNC mill is based on German-built hardware, but the CNC electronics were installed by a US company. Hence it used Mach3, ESS, and Clearpath servos - all US-sourced, of course. However, Mach3 has been obsolete for years,and there have been issues about poor levels of support for the ESS. So when I started getting data buffer errors, there seemed to be little point in trying to diagnose them and I scrapped the lot and replaced them with a UC300ETH and UCCNC - both of which are current and fully supported (and European!). I haven't had a problem with that setup, although I do use Win10. Unfortunately my Mach3/ESS setup did not have a slave axis so I have no reference settings to compare with you.

One odd thought - you are using a fully-licensed copy of Mach3? There are known issues with some copies of Mach3 from less than scrupulous suppliers which manifest themselves in all sorts of strange ways. Maybe worth a re-install of Mach3 and if necessary using a trial licence, to ensure that you have "clean" software? Apologies if your copy comes from an impeccable source!