This happened to me and I found that , while it was a bit tricky to re-add the logo using the pages UI, when I reloaded my page in the old power apps portals interface m, it gave me the option to readd the logo image more easily. So, instead of editing the portal in make.powerpages.microsoft.com I switched over to make.powerapps.com.
This answer may only be valid while both UIs are available to access the same portal
