i was messing around with a barb blood hunter and would see what would happen if i activated the mutagen potency.

i have 17 strength without and activating it should give me +3 to strength.
Nothing happens with my strength.
shouldn't it raise to 20 and a max of 23?

or am i wrong believing that D&D beyond should calculate this for me?

The character sheet is not currently built to handle any temporary effects. Spells like Aid and Haste and effects like exhaustion also do not show up on the character sheet. General forum advice is to make a homebrew item like a ring that has the desired effects which you can equip and unequip as needed.

clever. thanks for the help

any chance you know of a how to? because i have no idea how to create this. found the menu but makes no sense to me.
found someone who already made it but i have to subscribe for beyond to add home brew

Check out the homebrew forum for some tutorials; my suggestion is to start by duplicating an existing DDB-built item that has some of what you need. So if you're trying to raise your strength, using the Manual of Gainful Exercise might be a good place to start, versus the Ioun Stone, since the manual also raises your max strength. Make a copy of that, see what its properties are, and you can probably figure out what you'd need to adjust.

Here's a video showing you how to make the item pocketmouse suggests.