Here is the transcript between eglot and my developing
haskell-language-server:

[jsonrpc] e[06:33:30.460] --> textDocument/signatureHelp[192] 
(:jsonrpc "2.0" :id 192 :method "textDocument/signatureHelp" :params
          (:textDocument
           (:uri
            "file:///home/linj/code/origin/hydra-scraper/src/TestHls.hs")
           :position (:line 107 :character 19)))
[jsonrpc] e[06:33:30.471] <-- textDocument/signatureHelp[192] 
(:id 192 :jsonrpc "2.0" :result
     (:activeParameter 0 :activeSignature 0 :signatures
                       [(:activeParameter 0 :documentation
                                          (:kind "markdown" :value
                                                 "\n\nFun doc for  `fsimple` .\n\n")
                                          :label
                                          "fsimple :: forall a. a -> Bool -> Char"
                                          :parameters
                                          [(:documentation
                                            (:kind "markdown" :value
                                                   "\n\nThe first arg.\n\n")
                                            :label [21 22])
                                           (:documentation
                                            (:kind "markdown" :value
                                                   "\n\nThe second  `Bool`  arg.\n\n")
                                            :label [26 30])])
                        (:activeParameter 0 :documentation
                                          (:kind "markdown" :value
                                                 "\n\nFun doc for  `fsimple` .\n\n")
                                          :label
                                          "fsimple :: Integer -> Bool -> Char"
                                          :parameters
                                          [(:documentation
                                            (:kind "markdown" :value
                                                   "\n\nThe first arg.\n\n")
                                            :label [11 18])
                                           (:documentation
                                            (:kind "markdown" :value
                                                   "\n\nThe second  `Bool`  arg.\n\n")
                                            :label [22 26])])]))

> Please see https://joaotavora.github.io/eglot/#Reporting-bugs-1 for how to 
> report an Emacs -Q recipe (including a LSP server setup).

Thanks for the link.  I forgot to check that it before sending my
report.

I do not have time to provide more info now.

I think the above information is probably enough for this bug.  In
particular, the above server reply of signature help should be enough
for reproduction.  If it is not the case, please let me know and I will
provide more info later when I have time.

[1]: e33c0a549153fa3894f3b5e9c5e42ce07a1a68c7
[2]: https://github.com/haskell/haskell-language-server/pull/4626