I will say, If you are pressing the transfers yourself you can simply wipe the face of the transfer after peeling then with a damp cloth wipe the face of the transfer and press it the 2nd time. Problem gone.
It's one of those situations where there are multiple variables than can attribute to it. Too much ink can cause it as well, It creates a longer dwell time.
Overall you have a wet ink that needs to be heated to evaporate the oils in it, even hours later after printing it without curing. Once it's heated you have this layer of adhesive that seals in the oil if it doesn't evaporate entirely before the transfer starts cooling. You'll see pinholes if you are curing too hot. For me for example, my prints run through a roll printer/shaker. each print is in the curing portion about 5.5 minutes at 230-240 degrees with the extraction fan pulling air out, causing it to cycle more often.
This is more than enough for a proper cure, but If I cut that time in half or even 1/3 when running a faster print mode I will run into the same issue and will need a shaker with a longer tunnel dryer to counteract the speed it's running through so it stays under the cuing portion for the same amount of time.
The literal jist of it is, the ink hasn't fully cured. The oily substances are from the ink seeping through the print. Not water, Not humidity. Any experienced printer will say the same. Generally you are going to test your prints and get your ink layer right before worrying about the cure, so given the ink layer is the least it can be for CMYK and white and be correct and opaque when pressed, you then move on to curing and getting those times down right. Any change can cause adverse affects on the entire system. It's why faster machines usually run a pre-heater and a longer tunnel heater so they can fully cure the prints when they print 12" in of film in 2 minutes. so you are going to end up with a 2-3 foot long tunnel dryer to maintain that 4-5 minute dwell time.
Think of it like microwaving food. It gets hot on the edges, sometimes even burns. The center is still cold. You have to let it sit on the counter for the heat to " seep" in and finish cooking it properly in the center. The same way people get " well done" steaks without overcooking them. The difference in this scenario once the film cools, that's it. It takes seconds. So you must have it dwell in a lower temperature setting for longer to heat the ink causing the glycol to evaporate instead of stay behind so it gets an even thorough cure instead of a quick edge cure.
Using an extraction fan would change this variable dramatically, because the heat coming off the bulbs are over 500F and the " ambient" temp is what the machine reads. So using a fan causes it to cycle more often, allowing more direct heat to cure the prints. Without the fan, the basic " ambient" temperature may remain within the set temperature, but the print not cure correctly due to the lack of direct heat. So it should cycle the bulbs on more often, for less time to counteract these effects, while also removing the toxic fumes. It's a win-win.
But, for sure you need your ink layer correct first. Start with CMYK, use the least you can without the colors being affected, then make an opaque white without it looking like a " bubble" on the film. make an ICC profile at those settings to get proper colors. You'll now have a good ink layer, not too little and not too much with good color accuracy.
Move on to curing then, because if not you'll have to figure out how to cure again once you change your ink settings again. It's not every time, but most of the time the change would allow it to cure faster or slower, it also affects how much powder sticks to the prints.
It's definitely a process, making it print and look okay is easy, getting everything to work together for the best possible outcome consistently over a period of time- is not and takes some patience and testing. Vibrancy, durability, hand feel, things like oily prints, prints peeling off the transfer, etc are all because the system isn't set up to work together yet. So me personally, I always start at step one, and move forward until It's all set to each other to provide a soft, vibrant, durable transfer that is the same month after month with minimal adjustments. Changing your film, inks, or powder will require you to adjust these settings again to some degree, sometimes even batch to batch if you use an inconsistent supplier.
But the most basic answer is, It's under cured.