In OH the REGEX must match the full document. The first matching group is what gets returned. Your expression only matches a part of one line of the document. It might be as easy as adding .* to the beginning and end of your expression.
2021-10-13 17:18:58.979 [WARN ] [.profiles.RegexTransformationProfile] - Could not transform state '<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN"
"http://www.w3.org/TR/html4/loose.dtd">
<html>
<head>
<META HTTP-EQUIV="Refresh" CONTENT="30">
<title> Address List </title>
<style type="text/css">
body { background-color:#C0C0C0; color:#0000FF; }
</style>
</head>
<body>
<h2>Liste der ausgewählten FG-Adressen:</h2>
----------------------------------------------------------------------------------------------------
<br><pre><b>TL300E01B01 </b>Aussenluftlufttemperatur Analogeingang mit Grenzwert
<br>13-10-21 18:20:57 <a href="fga/TL300E01B01 " target="_blank">12.43781 </a>
<br></pre>
</body>
</html>
' with function '.*\/TL300E01B01.*?[>](\d*.\d*).*' and format '%.1f'
with default it works, but I have then all that digits I do not really need.