The main cause of it is because an unstable mesh network of the Bee to the main M2 Standalone. Simply move the Bee closer to the M2 will help fix the problem you are experiencing.
If you are confused that why this question never happens before v1.2.1, that's because before v1.2.1 that was both 2.4G and 5G communication, but we find that if the Bees communicate in 2.4G, that may slow down the network speed, therefore we changed the band communication to 5G only to make sure the network speed of both the M2 and the Bees; but as 5G band can't have the wide range extension as 2.4G, even in the same location and distance as before, the light might blink in green, however, the speed part will improve compare to previous speed.
The reason is: we support 2.4G & 5G communication for mesh communication before, while some users found there are too many 2.4G Wi-Fi around, and the performance suffers from the crowd. It is nontrivial to change the channel for all mesh nodes. So, we tried to keep only the non-crowded and faster 5G only, however, this will lead to smaller coverage because 5G cannot spread as far as 2.4G. This is actually a trade-off between coverage and performance. We are still conducting extensive field tests before finalizing the configuration. At present, please move the 3rd bee near to anyone of other nodes to establish a green connection.
If you are unhappy with the blinking green, simply move the Bee closer to the M2 will help fix the problem you are experiencing.