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();
```
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
```
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.