Odd UniFi UAP behaviour

Hi, I have a network here at work it has a modest number of UniFi AC-Lites and AC-LR APs on it. We also have a cupboard full of older UAP's, the originals.

The network has two SSID's both on seperate vlans, and the AC WAPs work fine, no problems.

But the issue is the UAP's, if I have more than one SSID on them, then the clients will connect, then disconnect after about 10sec and will drift off to a distant AP that is too weak to provide reliable service. If I have just a singe SSID on these older WAPs the client will happily connect and remain so indefinitely with good reliability. Current firmware is 4.0.14.9736, controller is 5.10.17

Anyone else seen this odd behaviour?

Cheers

Cyril