I use structure blocks very frequently, and I had put in my usual settings, saved, loaded and for some reason this time the loaded structure was without slabs, stairs, doors and trapdoors, maps, fences, item frames, lanterns and glass panes. Unusually, in my opinion, only half of the signs were still present, and my customised signs had no text. As seen in one of the pictures, I tried a setup with stairs straight afterwards, but it worked as intended.

Unusual elements of this copy were that I had a customised sign from before 1.15.2 and 6 customised maps by replacing the .dat files, but I've never had any issues with either of those before, and I have used structure blocks on these elements before.

You can see the 'save' and 'load' parameters in the attachments. I've checked these settings and retried the process ~5 times but with the same result.