I can reproduce the issue on a C source file with clangd:

emacs -Q
M-x eglot
(setq flymake-show-diagnostics-at-end-of-line t)

```
unknown_function
void func();
```
[image: Screenshot 2024-10-18 at 21.06.43.png]


I can also reproduce on a elisp file without using eglot or any additional
packages:

emacs -Q /tmp/test.el
M-x flymake-mode
(setq flymake-show-diagnostics-at-end-of-line t)

```
;;; test.el --- Test for flymake diagnostics -*- lexical-binding: t -*-
;;; Commentary:

;;; Test file

very_long_variable_name_to_make_flymake_diagnostics_on_multiple_line

;;; Code:

(provide 'test)
;;; test.el ends here
```

[image: Screenshot 2024-10-18 at 21.02.10.png]

Make sure that the diagnostic is displayed on multiple lines (resize your
window if necessary). Place your cursor on
`very_long_variable_name_to_make_flymake_diagnostics_on_multiple_line` and
then press down.