Thunderbir 45.1.1 does NOT marking messages as read

Hi,

I recently upgraded from Thunderbird 38 portable to Thunderbird 45.1.1 portable.

Now it is no longer automatically marking the message as read after they are open.

The settings at OPTIONS -> DISPLAY -> ADVANCED ->
"Automatically mark messages as read" is properly set.
I tried different settings, like "Immediately on display" and "After displaying for X seconds".

At "About:config" I found some keys, that are also properly set:
mailnews.mark_message_read.auto: true
mailnews.mark_message_read.delay: false
mailnews.mark_message_read.delay.interval: 1

I still can mark as read/unread by right-clicking the message and selecting "Mark -> As Read" or "Mark -> As Unread".

For me this seems a bug.
Any idea on how to fix it and having messages automatically marked as read again?