You can store up to 2147483647 items in the slot.

If you use Shift+Click to put 64 glass bottles in all the slots of the brewing stand, then you can put 253 items under the cursor. If a friend or a 2nd account or a command block will replenish the bottles, then you can place up to 2147483647 items. Command for the command block: /data merge block ~ ~1 ~ {Items:[

{Slot:0b,id:"minecraft:glass_bottle",Count:127b}

,{Slot:1b,id:"minecraft:glass_bottle",Count:127b},{Slot:2b,id:"minecraft:glass_bottle",Count:127b}]}

How to solve:
1. Add a check for the number of items in the "holding slot".
2. Instead of 4 bytes in RAM, use 1 byte. This will lead to the fact that with future bugs, it will be possible to stack only up to 127.

https://www.youtube.com/watch?v=UOyh5X6pqb8 - bugs.mojang.com: the file size is more than 10 MB.
https://www.youtube.com/watch?v=uE7xO3eoH8M - 2:51