it's probably a CRF

https://en.m.wikipedia.org/wiki/Controlled-release_fertiliser

it probably is slowly releasing regardless of what the plant is doing

It really depends on the fertilizer used. Slow release fertilizers (not the same as controlled release fertilizers) are more reliant on soil moisture to make them available. Controlled release ferts tend to be more temperature dependent for availability (do not release nutrients at low soil temps (below 60F).

So whether or not you need to reapply in spring would be determined by how moist the soils was over winter or how warm the plants were kept. Dry and cool, probably no need :-)

It may be best just to see how the plants respond to spring and then fertilize as needed.

i dislike soils with fertilizers added unless I add them myself. That way, I know what is getting what and when. I do use a CRF on all my permanent containers on an annual basis in spring and when I pot up new plants. It is a 6 mo. release that is temperature controlled so that gets me through the primary growing season in good stead.