FPUs in the early x86 family are weird. They were typically on separate chips so you could have an 8088+8087, 80286+287, 80286+287XL (which was actually a 80387), 80386+387 (SX and DX models for 24 or 32 bit bus), 80386+287[1], 80386 or 486+Weitek[2], 80386+Weitek+387, 80486SX+80487 where the co-processor was a full CPU that disabled the main chip. And then there were the clones doing creative things such as the Nx586+587[3] which because of it's lack of on-board FPU was often confused for a 386 by software and lost the advantage of its Pentium ops.
So I'm not surprised the exception handler is a mess. It's a domain built entirely out of corner-cases.
[1] https://old.reddit.com/r/retrobattlestations/comments/hj12ck...
[2] https://micro.magnet.fsu.edu/optics/olympusmicd/galleries/ch...