Interesting. I haven't heard of this issue. Do you have a reference, by chance?
I did find one comment, more or less related, online at
https://www.propanecarbs.com/questions.html
Problem:
Engine won't start or is hard to start.
Solution:
Propane and natural gas require from 50% to 100% higher temperature to ignite a charge as compared to gasoline. The ignition must be in good shape. You might try changing the spark plug gap to around .025.
In other words, it needs a hotter spark to ignite it - but that doesn't say how hot it burns.
and
http://www.propane-generators.com/faq%27s.php
Misinformation Often Received
PROPANE WILL MAKE THE ENGINE RUN HOTTER AND BURN THE VALVES!
This is not true! Propane is actually lower in Btu's than gasoline. When set correctly the engine will run just as cool as gasoline. As with all fuels, proper air/fuel mixture settings are important.
Neither one of these sounds real scientific, and both of them are selling conversion kits, so a grain of salt is in order. I would welcome better info. However, although Honda doesn't appear to sell conversion kits, the Honda web site doesn't mention propane conversion, nor does their warranty. Considering the number of conversion kits sold, I would think that conversion-failures would be common enough to raise red flags.
Bill