I haven't examined all the code but note that the embedding code is within this:
<div><div id="593414789117836114" align="left"
style="width: 100%; overflow-y: hidden;"
class="wcustomhtml">
<a id="nabblelink"
href="https://forum.thstofficial.com/">
Tottenham Hotspur Supporters' Trust</a>
<script src="https://forum.thstofficial.com/embed/f1">
</script></div>
I suspect that the div with the long numeric ID is styled to have a fixed height. You'll also note that any overflow in the vertical plane will be hidden.
I don't think this is a Nabble issue and is determined by the way you have set the space for the forum on the host page.
I would turn to the help for your host site software.
Volunteer Helper - but recommending that users move off the platform!
Once the admin for GregHelp now deleted.