Prevent merge of elements when they are unique

Question

I'm using TinyMCE 6 and I've created a custom block insert using a <ul>, and for the most part this is working great. However, if I have several and position the cursor in front of one of the blocks and hit backspace, it then merges all the contained <li> into a single <ul>. I've tried using attributes like data-id, id, name etc but Tiny simply discards this data from one of the elements and does the merge anyway. If I use a random number as a class name, they do not merge so this is a workaround.

For reference, the blocks are set as noneditable (I have a double-click handler configured for a custom function), so I don't see why it would try and merge them anyway.

Any ideas?

Here's a snippet of HTML as it should be:

<ul id="123" class="leaders mceNonEditable">
<li class="item-en text_gold"><span>Mmm Cookies</span><span>$MONEY$$$$$$</span></li>
<li class="item-ja"><span>SOMEJATEXT</span></li>
</ul>
<ul id="234" class="leaders mceNonEditable">
<li class="item-en"><span>Mmm Cookies</span><span>$495</span></li>
<li class="item-ja"><span>SOMEJATEXT</span></li>
</ul>

And some that ended up:

<ul id="123" class="leaders mceNonEditable">
<li class="item-en text_gold"><span>Mmm Cookies</span><span>$MONEY$$$$$$</span></li>
<li class="item-ja"><span>SOMEJATEXT</span></li>
<li class="item-en"><span>Mmm Cookies</span><span>$495</span></li>
<li class="item-ja"><span>SOMEJATEXT</span></li>
</ul>