agreed- most likely scenario is candied (crystallized) honey. Easy to test- take a frame out and visually inspect- you may be able to see that the honey is candied. If you can dig some out of a cell and look at and eat it. You will know if it has candied.
What is not so easy is dealing with the issue. We had limited success taking the box off and using an incandescent light bulb attached to a thermostat to heat the entire box (placed inside a larger box with the light bulb and covered in blankets) to around 43C and hold it there for 24 hours or so. That did melt a lot of the honey- but not all. Afterwards we had to wash out the remainder by soaking the frames in warm water.
Alternatively bees will eat the honey- but it takes them a long time to eat candied honey from the frames. I did this once placing the frames near an active hive inside a box with a lid on and access for the bees. It took them maybe ten days or more to eat all the honey. There is a real risk of setting of robbing- or causing other issues doing this and I am not entirely sure it is worth it- vis-a-vis- the risk to benefit ratio.
Whatever you end up doing- be sure to clean out all the honey as if you leave any crystallized honey in frames- it will possibly act as a ‘seed’ and set of a chain reaction causing later honey deposited in those cells to crystallize rapidly too… This happened to my mothers hive last season- there was some candying coming out of winter- and later there was candying during autumn- which normally doesn’t occur. Crystal seeds that survived harvests in spring caused the honey to candy later in the season. (at least that’s what we think is most likely)
This issue is one of the main reasons we have taken to removing our flow supers over winter- the risk of candying honey is too great.
Any time the bees are not working the super much for long periods- or during cold weather- or when collecting things like canola- the risk of candying is there. And it is annoying! The moral of the story is:
only put the supers on when the bees are very active and actively use it - if they ignore it for long periods- take it off. Even if this means harvesting partially filled frames that are uncapped- do it. If you leave that honey for long periods it can candy. You can freeze it and feed it back to bees if it is unripe. Better to cut your losses at this point.
harvest as soon as frames are ready. Do not leave honey sitting for long periods if you can help it. This is especially important if they are foraging on anything that is known to candy fast- like canola/rapeseed.
if it happens anyway- be sure to remove all the crystals by thoroughly soaking the frames in warm water- then air dry them.
consider insulating your roof if you live in a climate with cold weather.