I was using 0.1.2 and then cloned and built and ran b34242560e361d0fd73a9f49e4be7052b96cde5e which still has the same problem. Looking at the patch if I add:

if m != pv.part.MIMEType+"/"+pv.part.MIMESubType {
    switcher.selected = i // Add this line
    continue
}

Then if I view an email with text/plain and text/html it works. I don't know much GO but hopefully this is helpful to solve the issue

Thanks for building such an awesome email client! I've been trying to get the configuration in aerc.conf to prefer html over plain text so I changed the alternatives line from:

alternatives=text/plain,text/html

to

alternatives=text/html,text/plain

and reloaded aerc but is still defaulting to viewing emails in plaintext. Is this a bug or am I not doing something wrong?