I have 3 sg550xg in a stack, with sg350x edge switches. The 350s are each connected by LAGs (2 members to each lag, ie xg1/3 and xg2/3) using bi-di FS.com SFPs.
Plugging and unplugging the cisco 350s is normal, pings are ok, no timeouts, acts like you would expect. Those are lag 3 and lag 21 in the logs. po3 and po21.
But if I plug in my industrial BlackBox LIE1014a, (lag #15 or po15) with any combo of SFPs (although no cisco brand sfp, we aren't rich) we get a drop. And, if we use the BlackBox brand SFPs it will crash whatever SG550 unit it is plugged into, requiring a reboot.
The STP status page says both my cisco 350 edge switches enter a "discarding" state. The BlackBox stays "forwarding". (screenshot below)
The RAM log says the po3 and po21 say "STP status blocking" when the the Blackbox is plugged in also. (screenshot below)
We have tried adjusting auto negotiation settings, not using LAGs, different combos of LACP, different SFPs. I havent yet played with STP settings, as I don't have much experience there.