Original bug report for Mustang:viewtopic.php?f=94&p=229852
Now, with update 1.4 the bug has spread to P-40. I suspect it's caused by "Auto C&D". Auto C&D might move the graphical representation of mixture lever to cut-off but the engine behaves as if the lever was placed in auto-rich. This means, you can leave it in cut-off and stop priming after it catches. It runs. You can fly it for a thousand miles without removing mixture lever from cut-off position. It does seem to use fuel regularly so you need to choose a tank where it takes the fuel from.
It might be possible to avoid the bug by disabling "Auto C&D" and using "Cold start" (cold & dark) option manually before each flight. At least my preliminary tests show the bug going away by disabling "Auto C&D" and doing cold and dark manually before flight. (You cannot set "Auto C&D" and fix the bug by applying manual C&D on top of it.) This fix also applies to P-51D Mustang with Accusim.Spitfire might or might not be affected because it has no fuel cut-off position for mixture. Since even the "slow running cut-off" switch was (if I remember correctly) some choke type engine brake rather than mixture cut-off... and probably the reason why you had to run the carburetor dry before you pulled the cut-off - to not flood the intake with flammable mixture. Ok, I'll stop ranting off-topic. Also I'm not that familiar with Merlin as I pretend to be. :p
My unofficial recommendation (as I'm not part of A2A) would be:
if you care for realistic start-ups, disable "Auto C&D" and use cold & dark manually prior to each flight. Enable it after 1.5 update (or 1.41 hotfix) has been released, hopefully the bug has been addressed by then.