Text editor changes Paypal code

When pasting Paypal code for a Buy Now button with multiple options into the text editor (not visual), WordPress adds extra <select> tags between the option values.

Affected code directly from Paypal:

<tr><td><input type="hidden" name="on0" value="Fall Con Registration">Fall Con Registration</td></tr><tr><td><select name="os0">
	<option value="Registration">Registration $80.00 USD</option>
	<option value="Sponsor/Adult Registration">Sponsor/Adult Registration $50.00 USD</option>
	<option value="Youth Staff Registration">Youth Staff Registration $60.00 USD</option>
	<option value="Bus Fee">Bus Fee $65.00 USD</option>
</select> </td></tr>

Affected code once I save or switch to the Visual editor and back again:

<td><input name="on0" type="hidden" value="Fall Con Registration" />Fall Con Registration</td>
</tr>
<tr>
<td>
<select name="os0"><option value="Registration">Registration $80.00 USD</option></select>
<select name="os0"><option value="Sponsor/Adult Registration">Sponsor/Adult Registration $50.00 USD</option></select>
<select name="os0"><option value="Youth Staff Registration">Youth Staff Registration $60.00 USD</option></select>
<select name="os0"><option value="Bus Fee">Bus Fee $65.00 USD</option></select></td>
</tr>

This makes multiple drop downs appear on the button instead of one drop down from which a user selects one option.

The only plugin I have that should make any difference is TinyMCE Advanced, and deactivating that does not change this behavior.

I have not had this problem until about a week ago, when I assumed the code was wrong on Paypal’s end. I was able to delete the unwanted tags with no problem then. Today, however, deleting the extra <select> tags doesn’t stick – they keep coming back. Yes, I’m getting the opening and closing tags when I delete. And I’ve double checked – Paypal’s code is fine on their end. The change happens in the Text Editor.

It also happens on different sites with different configurations – I’ve tried the same code on three separate sites. The behavior is the same for both 3.9.1 and 3.9.2. I’m don’t know about 3.9 as I don’t have any sites running that version or earlier.

I have to be able to use the Visual Editor on a post with Paypal code in it! Any ideas what’s going on here??

Edit to add:

There is a bug report on this, but the unintelligible comments say that it is “fixed” without telling how to fix it. Little help?