Thanks both for the extra input.
On signal level: as noted in my post above, both sensors show “Excellent” for signal and battery (firmware 0.0.0_0005). On the environment: both sensors are in the same room as the G350, with no wall between them at all - 2.5m and 2.7m away. So this is actually a more favorable setup than the two-wall case described. The only other notable third-party smart devices I have are Philips Hue LEDs and Tado V3+ stations - nothing unusual or unexpected.
Given all that - excellent signal, same room, no wall, fresh batteries, re-pairing done - I think I’ve reached the limit of what I can reasonably test myself. These are consumer-grade devices, and I’m not going to spend several days troubleshooting further. This lines up more with coccolino_26’s theory: a G350 to T&H sensor communication issue rather than an environment problem - yet G350 + G100 + Cube T1 = 100% flawless.
I’m waiting on a reply from Aqara support, who emailed me directly. Thanks again for the help!