I've had this happen to me on multiple occasions and from my conclusion I see that the common items that result in a fluctuation from minor to drastic are the Shovel, Stove Cooking Pot with a Gas Canister equipped filled with water and a Plate Carrier with Pouches Equipped.

To begin, all three items must be in your inventory for the fluctuation to happen. The Shovel and the Stove Cooking Pot with a Gas Canister equipped filled with water, I'll call this Gas Pot for short, are what makes the player lose stamina. After months of testing I saw that rain is one of the common problems. Once drenched if either of the three items get equipped to hands stamina will either gain or drop. Again it fluctuates and can not pinpoint which will happen first.

First, when it rains equipping the shovel or Gas Pot will make you lose stamina. I can have it placed anywhere (usually on my back for the shovel) and will have the stamina stay the same meaning I'm still heavy as if I picked up some extra weight. Equipping both will make you lose and never gain.

On a separate occasion when taking out items from the Gas Pot you lose stamina as well. For example; equipping the Gas Pot you first separate the pot. Once the pot is on your hands you may remove the items. Doing so will gain stamina but either quick equipping or dragging the pot back to the stove will make you lose what you have gained possibly more.

Second, is the Pouches and the Gas Pot and what they can carry. When carrying the Gas Pot full of food or items if I place it in the pouches I can lose 25% of my sprint depending on what is carried in the Gas Pot. If the Gas Pot is placed in my pants, shirt/jacket or backpack I regain my sprint. This goes for other items such as car batteries, radiators and even small items to fit the 24 slot pouch. Keep in mind equipping the Gun Holster with gun will lower you stamina versus carrying them on the belt.

In the end, to fix these issues I wring my mask, armband, shirt/jacket, pants and I log off to the main menu and restart the official server. This occurrence will happen as well when you get out of a lake or a pool of water if the player has been submerged for too long. As for a fix with the Pouches and Gas Pot it is all about item location regardless of the weight and size.