If a message is previewed but not opened in Facebook Messenger, does the sender see it as "seen"?

I would suggest you to follow the tricks that have been explained in the following post. They mentioned an app named Unseenly to install, hope it works for you as well.

If you preview it by bot opened the app on your device but just look at it on the lock screen of an iPad, then it won't get marked as seen

If you preview it by not opening the message, in that case it will not show as "seen" since you haven't opened the message. 

You can download this app on Google Chrome which allows you to read messages without showing the "Seen"

No. If you see a message notification and you click on it you see a preview of the message. As long as you don't click on that (the unread message will be marked in a different color in the preview), the sender does not see "seen". They only see it as "seen" when the message is marked "read" (marked same color with other old messages in the preview).

As far as what I have experienced in these past days, I have no clue quite frankly and that no one else does either. Facebook is a screw up and cannot do anything correctly. So I would not follow the whole SEEn BS in a chat scenario. You will never know unless you get a personal message back from the person.

I had a friend who I sent a message to last night at around 11pm. I then opened the message and it said seen at the same time that I sent the message to her. I asked her, did you see my thank you message at the set time that it said SEEN/when I sent it. She told me no. She had her tablet on, but the screen was on sleep mode like it normally does when idle. She came back at 1am saying she just got my messages to her at 1am and she didn’t see the messages at 11pm because she was busy.

It is depend on the condition whether you read the message or not if you not read then the sender can’t see and if you already opened this message then the sender see it that you have read the message.