I don't think anyone is claiming that the -A version does not repeat (certainly not in this thread or anywhere I've seen on this forum; if people misread and are now saying elsewhere that they don't repeat at all, that is a different story). People are just saying that, like with most other ZigBee repeaters, Xiaomi devices do not work well with them. Furthermore, the Xiaomi devices use a custom ZigBee profile--they take a lot of work to get paired and sometimes stay paired with both ST and Hubitat, which of course use the standard HA profile, and both had to make firmware changes (guessing ST's may have even been accidental) before the devices actually "worked" on their platforms. Being non-standard, it's no surprise that some repeaters might have problems with them as well.
The biggest problem with Xiaomi devices seems to be with end-device timeout settings. The ZHA specs recommend checking in with the controller once every 60 seconds. It appears the Xiaomi devices only do this about once every 50-60 minutes (though note that the 60 seconds interval isn't a requirement, and many devices--especially sensors that don't usually receive information from the controller, don't check in every minute). There are additional issues with the Xiaomi devices in that if the controller/repeater's timeout is exceeded--which shouldn't really be a problem because the end device can request to rejoin--the Xiaomi devices also exhibit nonstandard behavior and don't complete the rejoin.
When people set up an XBee as a repeater, they are sure to set the end-device timeout (or "child device timeout" as XBee calls it) to a value greater than 60 minutes/360 seconds. I have no idea how Sylvania makes or configures their hardware, but a reasonable guess would be that their end-device timeout is set to less than 60 minutes, which causes it to think Xiaomi devices have dropped (and they won't successfully rejoin due to nonstandard behavior on the Xiaomi device itself, so keeping it joined is better). Sylvania wouldn't have to intentionally do anything to "drop" or "not repeat" Xiaomi devices--this is obviously not a problem unique to them, as most off-the-shelf ZHA devices also suffer the same problem (the earlier revision of their hardware being the rare exception, probably due to sheer chance of how they decided to set the timeout).
Of course, there could be other reasons, but Hubitat and Home Assistant/bellows users have, I think, both found the timeout to be the issue with their ZHA controllers (and Xbee users report it works well with homemade XBee repeaters--myself included), so it's a likely guess. They can read this thread or a couple others linked to here or in the ST Community forum for a wealth of information of what people have discovered about these devices.