I build with MRUBY_CONFIG=host-debug.
mruby revision is dcee404 .

p "①②③④⑤\xe2".size
# => 6
p "①②③④⑤\xe2".unpack1("H*")
# => "e291a0e291a1e291a2e291a3e291a4e2"
p "①②③④⑤\xe2".index("\xa4")
# => 6
## Expected to nil
## mruby-3.3 returns nil
p "①②③④⑤\xe2".split("\xe2")
# => ["", "\x91\xa0", "\x91\xa1", "\x91\xa2", "\x91\xa3", "\x91\xa4"]
## Expected to ["①②③④⑤"]
## mruby-3.3 is also wrong

The String#split method uses the result of mrb_memsearch() directly.
The String#index method adjusts the result of mrb_memsearch() with byte2char(), but the result is wrong.

Just reporting it for now.