If you used a promo code then you got some kind of promo bonus. Most promos give a bonus on each billing cycle for the first six months - a discount or some extra data, whatever - and most of these are conditional, you lose the ongoing promo bonus if you switch away from the promo plan. Doesn't matter if you switch plans after the promo bonus has already stopped.
Rewards/Points, Available Funds, Add-ons, and fixed Promotion items are attached to the account, not the plan. They don't fall off the account until they're consumed.
The details are always explicitly outlined on the announcement page related to the specific promo. Public Mobile can't really be blamed when their customers don't bother to read or remember the details.
It's good to know there's a warning, that you need to pause and decide and commit, that you can't inadvertently cancel your promo with a single impulsive or errant click.
