Underlining misspelled words no longer appear with neovim 0.9.0

[David Ross]

Describe the bug
Since upgrading to neovim 0.9.0, I see that misspelled words are no longer underlined when executing :hi SpellBad cterm=underline. However, it words perfectly well when using 0.8.3 with this plugin. When I switch to the default color scheme in 0.9.0, I see it works fine. Which makes me think it is a compatibility issue between 0.9.0 and this plugin.

To Reproduce
Steps to reproduce the behaviour:

1. In Neovim 0.8.3, open a txt file and add some misspelled words heere for example and save it
2. execute the following two commands

:setlocal spell spelllang=en
:hi SpellBad cterm=underline ctermfg=Red ctermbg=none

3. You should see misspelled word underlined like so
   
4. Switch to Neovim 0.9.0, open the same file and execute the commands denoted in step 2, and you will see the underline does not appear
   
5. Execute :colorscheme default and should see the underline appear under the misspelled word
   

Expected behavior
The underlining of misspelled words works properly across all compatible versions of neovim.

Screenshots
See above