@maboleth : You need to include the platform are you playing the game on, and the megaserver to which you connect, when you submit a bug report here. (Please review: Guidelines For Bug Report Forum)

When you say "logging off", I assume that you select "Log Out" from the ESC Menu while playing a character that has recently eaten Food to increase their Maximum Magicka capacity, and the software displays the Character Select screen. After you select the same character to resume play, you find that the food buff remains and the amount of Magicka displayed on the HUD Magicka bar includes the buffed Maximum. However, the length of the character's Magicka bar is not increased accordingly.

(1) In my own experience (playing on PC NA), first, I should wait until the network latency is no longer displayed in red digits, because the software is unlikely to accept my input while they are displayed in red. When it does accept my input while the digits remain displayed in red, the outcome of any action that I attempt while they remain displayed in red is often unpredictable and unreliable. Also any data which the software displays while the latency digits are in red can be -- temporarily -- incorrect.

(2) In my own experience, the length of one or more of the HUD Magicka, Health, and Stamina bars often appears to not be in proportion to how much capacity that the character actually has for Magicka, for Health, and/or for Stamina, respectively. Ordinarily the game software soon corrects the bar(s) to the proper size, however.

(3) Note also that when a player increases the distance between the "camera" and the character, the length of each respective bar decreases, and when a player reduce the distance between the "camera" and the character, the length of each respective bar increases. Frequently, the game software changes that distance to properly display the corresponding output on the HUD, without any player input. The appropriate corresponding change in the apparent length of the Magicka, Health, and/or Stamina bar, respectively, is not always smooth and simultaneously displayed.

However, if the size of the Magicka bar apparently never reflects the increase by that specific Food buff, after you have re-logged on to the character, then there could be a bug in the software.

The first thing to do if you are playing ESO on a PC or Mac is to disable any add-ons which have some relationship to the Food buff (e.g., Should I Drink or Eat) to see whether that restores the change in the size of the HUD Magicka bar to reflect the effect of the Food buff.