Do you know what repeaters they are, and have you used something like XCTU with an XBee to see which repeaters they were actually routing through (if any) on both ST and Hubitat? Just asking because it's possible the ST ZigBee radio (from the ST controller itself) was stronger than that from the Hubitat stick, so maybe your devices are using different routers on Hubitat compared to ST. Many are reported to have problems on Hubitat, which some ST users have also noticed. (This could especially be true if you paired them hear the hub was was recommended on ST but tried pairing in-place on Hubitat--you mentioned they're in hte same locations, so I'm not sure.)
I don't know why there'd be differences between repeaters on ST and Hubitat. My suspicion is that there really isn't, and I just don't read the ST forums much enough anymore to see that their users are equally aware. But I also suspect Hubitat users are better able to test and troubleshoot things like this. (But the Xiaomi devices themselves are definitely reported to "work" with both controllers, keeping in mind that we're doing something non-standard and it's just luck and careful hub firmware that keeps these working--and the repeater behavior should be the same regardless of controller, so it's hard to say without knowing how they were/are routing.)
In any case, besides the excellent suggestions above (including checking your channels), there is a lot of advice, including known good/bad repeaters, in this thread: Xiaomi devices - are they pairing / staying connected for you?. I'd check it out if you haven't already.