KMail first line of text in rich text emails

I’ve had this problem for a while now over several versions (updates) of kmail and googling it can’t seem to find others that have the same issue.
Hoping somewhere might have an idea . . .

When composing rich text emails (which is set as my default) the email appears in the composition as completely fine.
However, at the receivers end the first line of text is always one point smaller than all other lines.
You can probably see the issue in this screenshot
what might be the cause of this . . . possible solutions?

This is currently evident in KMail 20.08.0-1.2
If switching to plain text then the message is displayed fine at the other end

How does the raw received e-Mail look?

With kmail2 version 20.04.2 on Leap 15.2 it looks OK here.

The sent and received HTML should look something like:

<body><p style="margin-top:0;margin-bottom:0;margin-left:0;margin-right:0;"><strong>First line.</strong></p>
<p style="margin-top:0;margin-bottom:0;margin-left:0;margin-right:0;"><strong>Second line.</strong></p>
<p style="margin-top:0;margin-bottom:0;margin-left:0;margin-right:0;"><strong>Third line.</strong></p>
<br /><br /><br /></body>