Your issue was also bothering me and I found a dependency on the regional settings of the PC in the way that the notes client interprets HTML.
In your HTML code you have the font size defined with a decimal point (10.0pt), I noticed that Word does this also, and this breaks the processing in Notes if the OS is not configured with the dot as decimal symbol.
This might also explain why it is not related to Notes versions but depending on the user.
For the signature you can fix this by removing the .0 from the point sizes.
I have not found a description of this issue elsewhere and I’m still looking for a solution to this. We can control this for the signature but the problem is also with received emails and in our company we see now Exchange/Outlook users as we are migrating and their signature shows incorrect on notes depending on the regional settings of the recipients. This is not what people want to see.
With Release 8.5.2FP1 SHF179, when I attempt to change the Font size from ‘normal’ to ‘large’ or ‘extra large’ the content in the body of the message does not change in size. Text in all other data areas do change appropriately.
I wonder if it has something to do with the content being wrapped in html tags. If I send a message into my Notes account as plain text, all works as expected.