Spark needs to use the users presence prior to reconnect.
From the forums:
I find that Spark 2.5.1 returns to basic Available from "auto away" or server reconnects although I have a custom substatus configured and selected.
Could this substatus not be remembered locally and return to instead (I realise that custom substatus are not persistable server-side)?
Mass closing of all bugs resoved during 2.6.0 and 2.6.1