Hello,
The Wooden slab bed can't burn, same for Metallic and for Slab double bed.
By looking xml files, we found from where the issue come:
<statBases Inherit="False">
<MaxHitPoints>140</MaxHitPoints>
<WorkToBuild>400</WorkToBuild>
<Mass>30</Mass>
<BedRestEffectiveness>1</BedRestEffectiveness>
<ImmunityGainSpeedFactor>1.07</ImmunityGainSpeedFactor>
<SurgerySuccessChanceFactor>1</SurgerySuccessChanceFactor>
</statBases>
Inherit="False" without <Flammability>1.0</Flammability>, so of course it can't burn.
My guess is that Slab bed was thought to be made only from stone, then woody/metallic was added but flammability forgotten.
Thanks for the bug report, I have confirmed this in 1.3.3144. I'll have a report in for the devs to review!