The Lehr spec's are, as you stated, 2000 ± 150 RPM. 2150 RPM is, in my opinion, a very high idle speed but perhaps it is necessary to prevent it from cutting out as you noted. I think a call to customer service or your dealer might help you in rectifying this. Where did you buy the motor from and did they perform a pre-delivery inspection? There are some rumors on the internet that the propane they use in China to test the motors is less "potent" than the propane here in the United States and therefore many motors require both idle speed and mixture adjustments upon delivery. Motors bought direct from dealers (rather than online) are supposed to be adjusted by the dealer prior to delivery to the customer. A motor straight out of the box from China has not had these adjustments.
For now I've decided against the Lehr motors and I'm sticking with my gasoline powered 4 stroke. The propane technology is promising and I think there's a real market for propane powered outboards for use in applications where the motors are run infrequently and for short durations. I have a sailboat and infrequent, short use of my motor is the norm. I really only use it to get into and out of my slip and occasionally if the wind dies while I'm out on the lake. My main issues with the Lehr's are overall construction quality (including ill fitting cowls, cheap plastic and persistent oil leaks), repeated problems with idle quality such as yours (which seems not to affect every motor), and the lack of a robust dealer/service/parts network.
I think Lehr can get there eventually but only time will tell. I'll be really interested if Tohatsu, Honda, Yamaha, etc...get on board with propane especially for the US inland small boat market. But that market may not be lucrative enough to spur these larger companies to invest in that type of development.
All that being said, there are also lots of people that love their Lehr's. I think with some partience and some support from Lehr or your delaer directly that you will get there! Good luck and let me know how it works out.