UPDATED: July 5/18 including SD08's post.
First a word of CAUTION: the following analysis is based on personal experience and observing posts to this forum, and has not been confirmed officially by Public Mobile.
It was helpful to me in understanding these issues to think of PM as having two separate systems: System A. being one that takes money, and System B. one that provisions services.
ALSO, no explanation really suffices when you have paid enough for your plan AND have no service.
EXPIRED vs SUSPENDED
Just before midnight on the day your plan expires System A changes your account status to Expired and begins looking for money. At midnight your account status shows "Suspended". A few hours later all your Rewards are credited to your account as Available Funds and Rewards are reset. System A then compares the Rewards just credited to your plan cost (say $120). If Cost > Available Funds the remainder is charged to your credit card IF you have autopay turned on. At this point System A says to System B “We have $120” and System B compares $120 to your plan cost. Assuming $120 is equal or greater than your plan cost System B provisions your services.
On the other hand, if you are paying manually, or if your credit card fails, System A says “I have $100” and System B “Suspend services” because you’ve not yet paid $120. At this point your phone stops working EVEN though you have paid part of your plan cost. Your account status now becomes “Suspended”.
AVAILABLE FUNDS vs REWARDS
Took me quite a while to figure this out, but Rewards are not “Available Funds” until midnight on your expiry date. Until then Available Funds are ONLY credits or money you have paid.
AUTOMATIC PLAN CHANGE GLITCHES at Expiry
These seem to be abating. I think they arise because of miscommunication between System A and B triggered by System B updating to your new plan too late. There are two cases:
1. old plan was $120, new plan is $90. System A takes $90 and tells System B “We have $90”. System B has not yet updated and thinks you are still on the old plan. Therefore service is Suspended because $90 is less than $120.
2. old plan was $42 new plan is $120, so System A takes $120 and at midnight tells System B “We have $120”. System B says “great” but again provisions the old plan so service is not suspended and you have a credit of $78.
So in Case 1., the only foolproof way to avoid service suspension during an automatic plan change is to make a manual payment such that Available Funds plus Rewards at least equals your old plan cost.
The moderators can quickly (i.e. not during promo periods) clear up these automatic plan changes glitches.
Clearly I have too much time on my hands! 
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