Doing a quick search the other day, I came away a bit confused on the behavior of the sleep modes or the resumption behavior when exiting.
I’m running in manual mode.
I’m using system firmware 0.5.3. I notice when I resume for sleep that the wifi doesn’t reconnect (breathing white).
Is there a known issue with any particular version of the system firmware and sleep modes?
First you’d need to tell us what kind of System.sleep() you are waking up from and next, what state was the connection before going to sleep?
However, going with the most recent firmware version isn’t a bad idea either 
I noticed this coming from the stop sleep mode. I have the photon configured for a 5 min timeout, or trigger from a limit switch input.
It does appear to wake from the sleep mode (D7 LED goes from off, to breathing white). However, there is no Wifi connection.
I know, I know it’s old system firmware. I’m building locally, and ran into some hiccups when attempting build against later system firmware, so I put it off.
You can still test the wake behaviour with more recent firmware when building your test sketch in Web IDE.
But was your device connected and breathing cyan before Stop Mode sleep?
IIRC right after waking from Stop Mode sleep the same connection state should be reestablished that was present when going to sleep. But the older systems had a problem when you disconnected prior sleep but didn’t wait long enough to properly disconnect and/or switch the radio module off.