hide-lines doesn't work in org mode buffer

[tmalsburg]

Test file is test.org with content:

test

When I call hide-lines with regexpr test, nothing happens when the buffer is in org-mode. However, when I switch to fundamental mode, the line with test disappears. Back in org-mode, when I place the cursor on test and call C-u C-x =, I get this output:

             position: 3 of 4 (50%), column: 2
            character: s (displayed as s) (codepoint 115, #o163, #x73)
              charset: ascii (ASCII (ISO646 IRV))
code point in charset: 0x73
               script: latin
               syntax: w 	which means: word
             category: .:Base, L:Left-to-right (strong), a:ASCII, l:Latin, r:Roman
             to input: type "C-x 8 RET 73" or "C-x 8 RET LATIN SMALL LETTER S"
          buffer code: #x73
            file code: #x73 (encoded by coding system utf-8-unix)
              display: by this font (glyph code)
    xft:-PfEd-Fantasque Sans Mono-normal-normal-normal-*-23-*-*-*-m-0-iso10646-1 (#x54)

Character code properties: customize what to show
  name: LATIN SMALL LETTER S
  general-category: Ll (Letter, Lowercase)
  decomposition: (115) ('s')

There is an overlay here:
 From 1 to 5
  invisible            hl


There are text properties here:
  fontified            t

Tested with emacs -Q and GNU Emacs 26.1.91 (build 1, x86_64-pc-linux-gnu, GTK+ Version 3.22.30) of 2019-02-01 on Ubuntu Linux 18.04.