LibreOffice 5.1 regression see https://bugs.documentfoundation.org/show_bug.cgi?id=96245 — it appears this is to do with a new requirement that all draw:frame elements need draw:name properties in LO V5.1

broken.xml

<text:p>
	<draw:frame text:anchor-type="as-char" draw:style-name="fr1" svg:width="95%">
		<draw:text-box>
			<text:p>
				<draw:frame text:anchor-type="as-char" draw:z-index="1" >
					<draw:image xlink:href="Test1.png" xlink:type="simple" xlink:show="embed" xlink:actuate="onLoad"/>
				</draw:frame>
			</text:p>
			<text:p>Figure 1—Blah Blah Blah Blah Blah Blah Blah Blah Blah Blah Blah Blah Blah Blah Blah</text:p>
		</draw:text-box>
	</draw:frame>
</text:p>

<text:p text:style-name="Standard">Blah blah blah Blah blah blah Blah blah blah Blah blah blah Blah blah blah Blah blah blah Blah blah blah Blah blah blah Blah blah blah Blah blah blah.</text:p>

<text:p>
	<draw:frame text:anchor-type="as-char" draw:style-name="fr1" svg:width="95%">
		<draw:text-box>
			<text:p>
				<draw:frame text:anchor-type="as-char" draw:z-index="1" >
					<draw:image xlink:href="Test2.png" xlink:type="simple" xlink:show="embed" xlink:actuate="onLoad"/>
				</draw:frame>
			</text:p>
			<text:p>Figure 2—Blah blah blah</text:p>
		</draw:text-box>
	</draw:frame>
</text:p>

working.xml

<text:p text:style-name="Standard">
	<draw:frame draw:style-name="fr1" text:anchor-type="as-char" svg:width="0.041cm" style:rel-width="95%" draw:z-index="0">
		<draw:text-box fo:min-height="0cm">
			<text:p text:style-name="Frame_20_contents">
				<draw:frame draw:style-name="fr2" draw:name="1" text:anchor-type="as-char" svg:width="0.566cm" svg:height="0.566cm" draw:z-index="1">
					<draw:image xlink:href="Test1.png" xlink:type="simple" xlink:show="embed" xlink:actuate="onLoad"/>
				</draw:frame>
			</text:p>
			<text:p text:style-name="Frame_20_contents">Figure 1—Blah Blah Blah Blah Blah Blah Blah Blah Blah Blah Blah Blah Blah Blah Blah</text:p>
		</draw:text-box>
	</draw:frame>
</text:p>

<text:p text:style-name="Standard">Blah blah blah Blah blah blah Blah blah blah Blah blah blah Blah blah blah Blah blah blah Blah blah blah Blah blah blah Blah blah blah Blah blah blah.</text:p>

<text:p text:style-name="Standard">
	<draw:frame draw:style-name="fr1" draw:name="2" text:anchor-type="as-char" svg:width="0.041cm" style:rel-width="95%" draw:z-index="2">
		<draw:text-box fo:min-height="0cm">
			<text:p text:style-name="Frame_20_contents">
				<draw:frame draw:style-name="fr2" draw:name="3" text:anchor-type="as-char" svg:width="0.566cm" svg:height="0.566cm" draw:z-index="3">
					<draw:image xlink:href="Test2.png" xlink:type="simple" xlink:show="embed" xlink:actuate="onLoad"/>
				</draw:frame>
			</text:p>
			<text:p text:style-name="Frame_20_contents">Figure 2—Blah blah blah</text:p>
		</draw:text-box>
	</draw:frame>
</text:p>