What resin are you using, and could you describe your post-processing steps?
I assume the prints still look good after you remove them from the build platform (i.e. you’re not bending them out of shape in that step). I also assume they’re adequately supported (support density and contact size).
I wash in the Form Cure per the recommended settings from Formlabs. You want to shorten the time for parts that have small, thin, delicate features. I’ll often wash really delicate parts by hand instead, using the squeeze bottle. Then let them completely dry before curing in the Form Cure, again at the recommended settings (which vary per resin). Make sure you orient them in the chamber so the UV light hits them evenly (i.e. try to avoid one side getting persistently shadowed by another part).
The reason I suggest making sure the prints are complete dry before curing, is that alcohol swells them. You want them to un-swell before the extra curing locks the shape in place. In practice, you can get away with putting them in earlier, especially since preheating speeds the evaporation process, but I wouldn’t get too aggressive with that while you’re trying to troubleshoot issues.
Changing the alcohol every 15 washes sounds to me like it should be more than sufficient.
I assume you haven’t noticed any warping in your prints during the above steps or you would have said so.
The size, shape and orientation of your print can also contribute to warping after the fact. e.g. I made a large, bulky bracket out of a much earlier version of Tough resin. Even though it was super well supported, a large flat surface kept coming out slightly bent. After the print, the bend worsened a bit over time (just sitting on the desk without being installed anywhere). The older Tough resin wasn’t as stiff, which was part of the issue (the exact same print in Standard came out just fine). However, the shape and orientation also contributed. When the resin cures, the new layer shrinks a little bit which can introduce internal stresses that pull inward on the edges of your print and make them warp ever so slightly away from the rest of the layer. When the edges are always near the same area of the print surface, the forces can stack and exacerbate the problem. In my case I suspect the internal stresses remained to some degree even after the print was cured and made the warping a little worse over time.
Changing the orientation helped a little - doing so “swept” the active layer across the build area as the print progressed, spreading out the effect. Unfortunately it didn’t completely eliminate the problem (I was somewhat limited on orientations to choose from due to the size of the part) and I wound up going with Standard instead, although given material improvements since then I’m fairly convinced if I tried the same print in Tough v5 / 1500 / 2000 the adjusted orientation would work fine today. If you have the design flexibility, there may be other tricks you can do with the geometry to lessen the effect (e.g. try honeycombing or other patterns).
Finally, what are you doing with your print after it’s finished? e.g. If it’s flat like a pancake and you leave it out in the sun for a week, it will eventually curl upward as the top side undergoes additional curing but the bottom doesn’t. Also, most of the resins don’t excel under static load - e.g. Tough resin will “droop” over time from a sustained force. Depending on your application, other resins (e.g. Rigid) may be more suitable and offer enhanced rigidity true to the originally designed shape.
I’ve covered a lot of things that might contribute to warping, perhaps others will have additional suggestions. In the meantime, best of luck troubleshooting and resolving the problem! The folks over at Formlabs Support are pretty friendly and knowledgeable as well if you still have difficulty figuring it out.