The command "set lacp singleportlag enable" is only needed when we are initially trying to form the lag with a single port, with the intention to add a second port later down the line. Without this enabled, it would require two ports to link up for the lag to form. The behavior of singleportlag is only applicable to creation of the lag, not the operational aspect of it.
If you have singleportlag disabled, form a two port lag, and one of the cables gets disconnected, the lag will still stay operational on the single port even with singleportlag disabled.
Regards,
Ryan Careno