On ExtremeSwitching X440-G2 and X620 series switches, stacking is not stable with the original, un-patched ExtremeXOS 22.6.1 image (see xos0073370 in Resolved Issues in ExtremeXOS 22.6.1-Patch1-1). Standalone (non-stacking) operation is not affected by this issue.
To use stacking, upgrade to ExtremeXOS 22.6-Patch1-1.