They will come back when the agent collects data again.  We use multiple sessions to collect the performance data, inventory etc... in parrallel to each other.  Otherwise you would have to wait quite awhile in some environments before data would populate/update.
For our Foglight environment we have 2 virtual centers with both going to a single FGLAM. If I log onto vCenter and select sessions I show 4 connections to Foglight.If I kill the sessions they all come back.  Why 4 connections? I would expect to only have 1 on each vcenter.
vCenter session count