I worked on tracking this down for quite some time.

It seems that readmemb (and readmemh) may be broken?

I am wondering if you used SynplifyPro. I can't use that. In fact, Gowin now seems to recommend not using it and just using Gowin Synthesis.

readmemb (and readmemh) both seem to fail when the input file is too large. I found this out by making a smaller bitmap.mem which did work. I got it working for a 16x16 bitmap, but after that, even 16x18 failed.

If would be really interesting if you (or anyone) could try compiling with SynplifyPro and see if that works. If it does, then switch to Gowin Synthesis (really easy to switch) and rerun all the steps and program the Tang Nano. Then, does it just show a black screen? If so, I'd like to mention this to Gowin and see if they'll fix it.