Well, if things are really as you say (see the following) then it can only be a bug. Either that or what you said isn't true to the following points:

1. Your production after upkeep is 178k food and has remained that way for the entire duration of your offline time (reputation/production bonuses/townscape/upkeep reduction bonuses/VIP status didn't expire, nor did a big troop batch finish/was bought while you were offline).
2. You did not use your food prior to checking how much you had (start a new training/study/construction).
3. Your farm capacity (as opposed to what the Treasury capacity, trasury has no effect) is indeed above the amount of food you had logging in.
4. You were shielded and weren't attacked for the entirety of your offline time.

My guess is you missed something on #1. Since I have never seen any glitching with my production nor heard of anyone that has, I can only assume your detail on what really happened must be lacking.

Otherwise, why ask the question of "this shouldn't have happened for any explainable reason, why is it happening?" - the answer to such a question is obviously "a bug".

I have run a couple of tests by now by logging in and out of the game at various span of time and the results show a bug in the game. For some reason the production slows down when i log off.

Dear Lord,

Thank you for your message. Could you please clarify your question here? We want to help, but to avoid misunderstanding, it would be great if you could describe the situation in detail. Send us your ID number. To find it, tap the Menu, then "About the game". Thank you in advance.

Best regards,

Your Plarium Support Team