Pixelmator Pro PNG exports are losing transparency

My question is about Pixelmator Pro exports to png format. I use the app to draw textures for use in an app named Second Life. This involves exporting what I draw in Pixelmator Pro to png and then loading them into Second Life. All worked well until today. Now, if I export a texture with a transparent section, it uploads as opaque, not transparent. To test the problem I was having, I uploaded a texture I created on October 9. It worked fine. I then went to the Pixelmator Pro file that I exported on October 9 and exported it again. This time, the export (today) of the same file created a texture (a png file) that is not transparent where it was with the file exported on October 9. Everything else is the same. Export obviously has been broken.