Same here currently using PHP 7.2 and OST 1.5.2.
Issue: Disabling rich text editor, causes agent responses to show html markup. Client responses do not.
Workaround: Until it gets fixed, is to enable rich text. And disable rich text for client, by disabling with css.
There are many approaches to disabling it via css. My way was by creating an override css. Useful to minimize core changes, by fixing unforeseen fixes using only css (well some.).
In your client/header.inc.php insert a custom css at the end
<head>..
files/css/ost-overrides.css?cb6766e
...
</head>
Also, in your client/header.inc.php after body
<body>
<div id="client-pages">
In your client footer.inc.php, close it, after
<footer>..</div>
</div>
In your ost-overrides.css
#client-pages .redactor-toolbar-wrapper {
display:none;
}
and add any other client overrides, like so
/* Bugfix - Stop logo hover underlining */
#client-pages #header a { margin-bottom: 0; }
#client-pages #header a:hover { border-bottom: none; }
Hope this helps