If you have a stockpile where you limited the bins to 0, for example, and you click on the "custom" button to see what items are you permitting in that stockpile, the max bins allowed resets to default (the stockpile size).
#Storage (bin, barrels, wheelbarrows) quantity resets after clicking on custom in a stockpile
primal light
tropic field
Can confirm.
Clicking any type of stockpile (including Custom button which lets you view the settings) resets all settings of Reserved Bins/Barrels to default values.
Btw this is a duplicate of https://discord.com/channels/329272032778780672/1050794334775165058