Describe the bug
I've previously had similar issues- #157
Now by debugging (lsp-dart-get-sdk-dir) I've determined the cause of my issues.
(NB that flutter is added to my $PATH via a nix-shell and envrc)

Debugger entered--Lisp error: (error "Dart SDK not found inside flutter cache dir /nix/s...")
 error("Dart SDK not found inside flutter cache dir %s. C..." "/nix/store/i8r6cbgmrcpzhk3w4fqvvx18qm1y645q-flutte...")
 (if (file-exists-p dart-sdk) dart-sdk (error "Dart SDK not found inside flutter cache dir %s. C..." dart-sdk))
 (let ((dart-sdk (expand-file-name "bin/cache/dart-sdk/" (lsp-dart-get-flutter-sdk-dir)))) (if (file-exists-p dart-sdk) dart-sdk (error "Dart SDK not found inside flutter cache dir %s. C..." dart-sdk)))
 (progn (let ((dart-sdk (expand-file-name "bin/cache/dart-sdk/" (lsp-dart-get-flutter-sdk-dir)))) (if (file-exists-p dart-sdk) dart-sdk (error "Dart SDK not found inside flutter cache dir %s. C..." dart-sdk))))
 (if (lsp-dart-flutter-project-p) (progn (let ((dart-sdk (expand-file-name "bin/cache/dart-sdk/" (lsp-dart-get-flutter-sdk-dir)))) (if (file-exists-p dart-sdk) dart-sdk (error "Dart SDK not found inside flutter cache dir %s. C..." dart-sdk)))))
 (or lsp-dart-sdk-dir (if (lsp-dart-flutter-project-p) (progn (let ((dart-sdk (expand-file-name "bin/cache/dart-sdk/" (lsp-dart-get-flutter-sdk-dir)))) (if (file-exists-p dart-sdk) dart-sdk (error "Dart SDK not found inside flutter cache dir %s. C..." dart-sdk))))) (and (lsp-dart-flutter-snap-install-p) "~/snap/flutter/common/flutter/bin/cache/dart-sdk") (let ((result (let ((result (let ... ...))) (if result (progn (locate-dominating-file result "bin")))))) (if result (progn (file-truename result)))))
 lsp-dart-get-sdk-dir()
 eval-expression((lsp-dart-get-sdk-dir) nil nil 127)
 funcall-interactively(eval-expression (lsp-dart-get-sdk-dir) nil nil 127)
 command-execute(eval-expression)

So- if lsp-dart detects that the project root is a flutter project, it tries to find the dart sdk in (lsp-dart-get-flutter-sdk-dir)/bin/cache/dart-sdk. Due to how flutter is packaged in nixpkgs though, the dart sdk is not found there.
In my case, (lsp-dart-get-flutter-sdk-dir) evaluates to "/nix/store/i8r6cbgmrcpzhk3w4fqvvx18qm1y645q-flutter-wrapped/".
So in that case an error will be returned and lsp-dart won't try to find the dart sdk from $PATH
What complicated my issue is that one of my project repos didn't contain one flutter project, but a monorepo with multiple flutter projects in subdirectories. In that case, lsp-dart found the dart sdk from my $PATH instead. I was very confused about this, because I didn't get why the analysis server was started in one project but not the other.

Is there anything that can be done without unreasonable effort from lsp-dart side to support nixos better or is this on me as a user to fix?
one workaround is to find the dart sdk dir manually and set it in .dir-locals.el, but that seems quite involved. One reasonably simple way to find this would be to create a dart project and run (lsp-dart-get-sdk-dir) inside it.

To Reproduce
in a directory containing a nix-shell, .envrc and flutter project:

dir
|--shell.nix
|--.envrc
|--project
 |--pubspec.yaml
 |--lib/main.dart
 |--...

with a shell.nix like the following:

{ pkgs ? import <nixpkgs> {} }:
pkgs.mkShell {
 buildInputs = with pkgs;
 [
 flutter
 dart
 ];
 }

in emacs, using envrc and lsp-dart
(I use doom emacs with :lang (dart +flutter +lsp) and :tools (direnv))
open dir/project/lib/main.dart (activating dart-mode)
Expected behavior
dart_analysis_server starts in flutter root

Version
[LSP Dart] 1.24.2 at 2023年09月13日 @ Emacs 29.1
[Dart SDK] Dart SDK version: 3.1.0 (stable) (Tue Aug 15 21:33:36 2023 +0000) on "linux_x64"

[Flutter SDK] /nix/store/i8r6cbgmrcpzhk3w4fqvvx18qm1y645q-flutter-wrapped/
[Flutter project] true
[Project entrypoint] /home/nanashi/Documents/projects//lib/main.dart